Bug#51612 Un initialized locker_lost variable in pfs_instr.cc

Fixed the missing initialization of locker_lost.

This fix is not strictly necessary, but is desirable to re-align the code
from 5.5 and 6.0, and reduce the spurious code differences.

This will facilitate maintenance and help to apply patches cleanly, for merges.

storage/perfschema/pfs_instr.cc

@@ -66,7 +66,7 @@ ulong events_waits_history_per_thread;
 /** Number of instruments class per thread. */
 ulong instr_class_per_thread;
 /** Number of locker lost. @sa LOCKER_STACK_SIZE. */
-ulong locker_lost;
+ulong locker_lost= 0;
 
 /**
   Mutex instrumentation instances array.