bucket_engine.c:1413 core dump while possibly retrieving TAP stats during rebalance tests

I will look at the test logs later to find out during which test this crash happened
i think it is related to some new calls i added to the tests which get tap stats more ofte during test runs.

Thread 1 (Thread 15489):
#0 0x00c87402 in __kernel_vsyscall ()
#1 0x001efdf0 in raise () from /lib/libc.so.6
#2 0x001f1701 in abort () from /lib/libc.so.6
#3 0x001e926b in __assert_fail () from /lib/libc.so.6
#4 0x004e24de in must_lock (mutex=<value optimized out>) at bucket_engine.c:305
#5 0x004e2963 in bucket_get_stats_struct (handle=0x4e7200, cookie=0x9d26278) at bucket_engine.c:1413
#6 0x08055bd7 in transmit (c=0x9d26278) at daemon/memcached.c:6334
#7 conn_mwrite (c=0x9d26278) at daemon/memcached.c:5375
#8 0x08055ed5 in conn_write (c=0x9d26278) at daemon/memcached.c:5362
#9 0x0805b606 in thread_libevent_process (fd=10, which=2, arg=0x9cb3ff0) at daemon/thread.c:381
#10 0x0019b473 in event_process_active_single_queue (base=0x9cb42f0, flags=0) at event.c:1308
#11 event_process_active (base=0x9cb42f0, flags=0) at event.c:1375
#12 event_base_loop (base=0x9cb42f0, flags=0) at event.c:1572
#13 0x0805be97 in worker_libevent (arg=0x9cb3ff0) at daemon/thread.c:304
#14 0x00b3e5ab in start_thread () from /lib/libpthread.so.0
#15 0x00298cfe in clone () from /lib/libc.so.6