Spurious segfaults due to query metrics

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x13c)

    frame #0: 0x000000010c40f875 couchbase_impl.node`record_http_op_latency(char const*, char const*, lcb_st*, unsigned long long) [inlined] record_op_latency(op=<unavailable>, svc=<unavailable>, settings=0x0000000000000000, start=<unavailable>) at metrics-internal.cc:41:9 [opt]

   38

   39  	void record_op_latency(const char *op, const char *svc, lcb_settings_st *settings, hrtime_t start)

   40  	{

-> 41  	    if (settings->op_metrics_enabled && settings->meter) {

   42  	        lcbmetrics_TAG tags[2] = {{METRICS_SVC_TAG_NAME, svc ? svc : ""}, {METRICS_OP_TAG_NAME, op ? op : ""}};

   43  	        auto recorder = settings->meter->value_recorder_(settings->meter, METRICS_OPS_METER_NAME, tags, 2);

   44  	        if (recorder) {

Target 0: (node) stopped.

(lldb) bt

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x13c)

  * frame #0: 0x000000010c40f875 couchbase_impl.node`record_http_op_latency(char const*, char const*, lcb_st*, unsigned long long) [inlined] record_op_latency(op=<unavailable>, svc=<unavailable>, settings=0x0000000000000000, start=<unavailable>) at metrics-internal.cc:41:9 [opt]

    frame #1: 0x000000010c40f875 couchbase_impl.node`record_http_op_latency(op=0x0000000000000000, svc="query", instance=0x000000010314ed30, start=0) at metrics-internal.cc:62 [opt]

    frame #2: 0x000000010c41e7a2 couchbase_impl.node`lcb_QUERY_HANDLE_::invoke_row(this=0x0000000102f87880, resp=0x00007ffeefbff150, is_last=<unavailable>) at query_handle.cc:223:13 [opt]

    frame #3: 0x000000010c421061 couchbase_impl.node`lcb_QUERY_HANDLE_::~lcb_QUERY_HANDLE_(this=0x0000000102f87880) at query_handle.cc:555:9 [opt]

    frame #4: 0x000000010c4213be couchbase_impl.node`lcb_QUERY_HANDLE_::~lcb_QUERY_HANDLE_() [inlined] lcb_QUERY_HANDLE_::~lcb_QUERY_HANDLE_(this=0x0000000102f87880) at query_handle.cc:552:1 [opt]

    frame #5: 0x000000010c4213b9 couchbase_impl.node`lcb_QUERY_HANDLE_::~lcb_QUERY_HANDLE_(this=0x0000000102f87880) at query_handle.cc:552 [opt]

    frame #6: 0x000000010c40dbd1 couchbase_impl.node`timer_callback(sock=<unavailable>, which=<unavailable>, arg=0x0000000102f67e40) at timer.cc:43:5 [opt]

(lldb) f 1

frame #1: 0x000000010c40f875 couchbase_impl.node`record_http_op_latency(op=0x0000000000000000, svc="query", instance=0x000000010314ed30, start=0) at metrics-internal.cc:62 [opt]

   59

   60  	void record_http_op_latency(const char *op, const char *svc, lcb_INSTANCE *instance, hrtime_t start)

   61  	{

-> 62  	    record_op_latency(op, svc, instance->settings, start);

   63  	}

(lldb) p instance->settings

(lcb_settings *) $0 = nullptr

(lldb) p *instance

(lcb_INSTANCE) $2 = {

  cmdq = {

    pipelines = nullptr

    scheds = 0xd000000010318118 ""

    ctxenter = 8

    npipelines = 0

    _npipelines_ex = 0

    seq = 0

    config = nullptr

    cqdata = 0x0000000000000000

    fallback = nullptr

  }

  cookie = 0x0000000000000000

  confmon = nullptr

  mc_nodes = nullptr

  ht_nodes = nullptr

  cur_configinfo = nullptr

  bs_state = nullptr

  callbacks = {

    v3callbacks = {

      [0] = 0x0000000000000000

      [1] = 0x0000000000000000

      [2] = 0x0008000000000000 (0x0008000000000000)

      [3] = 0x0000000000000002 (0x0000000000000002)

      [4] = 0x0000000000000000

      [5] = 0x000000010c3ed460 (couchbase_impl.node`iops_lcb_dtor(lcb_io_opt_st*) at plugin-libuv.c:40)

      [6] = 0x0000000000000000

      [7] = 0x0000000000000059 (0x0000000000000059)

      [8] = 0x000000010c3ed4f0 (couchbase_impl.node`wire_iops2(int, lcb_loopprocs_st*, lcb_timerprocs_st*, lcb_bsdprocs_st*, lcb_evprocs_st*, lcb_completion_procs*, lcb_iomodel_t*) at plugin-libuv.c:630)

      [9] = 0x00000001031252b0 (0x00000001031252b0)

      [10] = 0x0000000000000000

      [11] = 0x0000000000000000

      [12] = 0x0000000000000000

      [13] = 0x0000000000000000

      [14] = 0x0000000000000000

      [15] = 0x0000000000000000

      [16] = 0x0000000000000000

      [17] = 0x0000000000000000

      [18] = 0x0000000000000000

      [19] = 0x0000000000000000

      [20] = 0x0000000000000000

      [21] = 0x0000000000000000

      [22] = 0x0000000000000000

      [23] = 0x0000000000000000

      [24] = 0x0000000000000000

    }

    errmap = 0x0000000000000000

    bootstrap = 0x0000000000000000

    pktfwd = 0x0000000101ce4d40 (node`default_loop_struct)

    pktflushed = 0x0000000100000001 (node`_mh_execute_header + 1)

    open = 0x0000000000000001 (0x0000000000000001)

  }

  kv_timings = nullptr

  pendops = {

    items = {

      [0] = 0x000000010315f460 size=4294967295

      [1] = 0x0000000101ce4d40 size=4294967295

      [2] = 0x0000000000000009 size=0

    }

    count = 0

  }

  wait = 53045520

  memd_sockpool = 0x000000010313b320

  http_sockpool = nullptr

  last_error = LCB_SUCCESS

  settings = nullptr

  iotable = nullptr

  retryq = nullptr

  scratch = ""

  vbguess = 0x000000010c3e93a0

  n1ql_cache = 0x000000010323cba8

  dcpinfo = 0x0000000101ce4f68

  dtor_timer = 0x100000001031667d

  btype = 0x1031149c

  collcache = 0xd000000010316b30

  crypto = 0x000000010c3eae10 size=4294967295

  deferred_operations = 0x000000010314a6b0 size=4294967295

}