Implement a few simple asserts in my_rwlock_t locks.

include/my_pthread.h:
  Declare two simple assert functions.
include/mysql/psi/mysql_thread.h:
  Add wrappers for new assert functions.
mysys/thr_rwlock.c:
  Add asserts.
sql/sql_base.cc:
  Silence a compiler warning for the case when
  SAFE_MUTEX is not ON.