Abort in ringbuffer_consumed() when stopping server or timeout

With the fix I provided for CCBC-274, stability of libcouchbase C client is much better in situations where a CB server is stopped. However, we still get incidental crashes on either stopping a server or timeout due to high load on the system.

Program terminated with signal 6, Aborted(gdb) where
#0 0x00007f7407bbf8e5 in raise () from /lib64/libc.so.6
#1 0x00007f7407bc10c5 in abort () from /lib64/libc.so.6
#2 0x00007f740997269a in ringbuffer_consumed (buffer=<value optimized out>, nb=41) at src/ringbuffer.c:263
#3 0x00007f7409973c5c in lcb_purge_single_server (server=0x7f73b0002208, error=LCB_ETIMEDOUT) at src/server.c:338
#4 0x00007f74099758d1 in lcb_server_timeout_handler (sock=<value optimized out>, which=<value optimized out>, arg=<value optimized out>) at src/timeout.c:38
#5 0x00007f7409b8cb44 in event_base_loop () from /usr/lib64/libevent-1.4.so.2
#6 0x000000000046e476 in con_thr (arg=0x1471218) at src/store_couchbase.c:2659
#7 0x00007f7407f27851 in start_thread () from /lib64/libpthread.so.0
#8 0x00007f7407c7594d in clone () from /lib64/libc.so.6
(gdb) up
#1 0x00007f7407bc10c5 in abort () from /lib64/libc.so.6
(gdb) up
#2 0x00007f740997269a in ringbuffer_consumed (buffer=<value optimized out>, nb=41) at src/ringbuffer.c:263
263 src/ringbuffer.c: No such file or directory.
in src/ringbuffer.c
(gdb) p n
$2 = 0
(gdb) up
#3 0x00007f7409973c5c in lcb_purge_single_server (server=0x7f73b0002208, error=LCB_ETIMEDOUT) at src/server.c:338
338 src/server.c: No such file or directory.
in src/server.c
(gdb) p req
$1 = {request =

,
bytes = "\200\224\000\r\004\000\001\314\000\000\000\021y\221\024\000\000\000\000\000\000\000\000"}

336 ringbuffer_consumed(stream, packetsize);
337 if (mirror)

So the crash is caused by the following abort in ringbuffer_consumed() :

lcb_size_t n = ringbuffer_read(buffer, NULL, nb);
if (n != nb)

(n = 0, nb = 41)
Apparently, ringbuffer_read() returning 0 instead of the number of expected bytes is possible in some situations and should not trigger an abort.