BUG#11384 drop database causes mysqld to core

Only reproducable in 5.1-wl2325, although the missing code is missing from 4.0 and up.


sql/slave.cc:
  Clean up after slave sql thread when exiting.
  
  Wasn't calling close_thread_tables().
  
  This was causing a crashing bug in the 5.1-wl2325 tree. It is conceivable that this
  could cause a problem in earlier versions, but we have not been able to reproduce.

sql/slave.cc

@@ -2721,6 +2721,7 @@ log space");
   net_end(&thd->net); // destructor will not free it, because net.vio is 0
   pthread_mutex_lock(&LOCK_thread_count);
   THD_CHECK_SENTRY(thd);
+  close_thread_tables(thd);
   delete thd;
   pthread_mutex_unlock(&LOCK_thread_count);
   pthread_cond_broadcast(&mi->stop_cond);	// tell the world we are done