safe_mutex: Trying to destroy a mutex keycache->cache_lock that was locked

RQG test crashes with assetion in the safemutex code

http://buildbot.askmonty.org/buildbot/builders/rqg-perpush-bugfix-tests/builds/25/steps/rqg_bugfix_tests/logs/stdio

the crash callstack points to "repartition_key_cache" function.

mysys/thr_mutex.c:608(safe_mutex_destroy)[0xc39f1d]
psi/mysql_thread.h:597(inline_mysql_mutex_destroy)[0xc1071e]
mysys/mf_keycache.c:1002(end_simple_key_cache)[0xc1066d]
mysys/mf_keycache.c:5342(end_partitioned_key_cache)[0xc17814]
mysys/mf_keycache.c:6109(end_key_cache_internal)[0xc184ae]
mysys/mf_keycache.c:6476(repartition_key_cache_internal)[0xc188ea]
mysys/mf_keycache.c:6527(repartition_key_cache)[0xc1898b]