decrement thead_count *after* THD is destroyed

because thread_count means just that: number of THDs
and shutdown code looks at it to know when to free
shared data structures that THD uses.

This fixes random crashes in ~THD on shutdown

sql/scheduler.cc

@@ -39,6 +39,7 @@ static bool no_threads_end(THD *thd, bool put_in_cache)
   {
     unlink_thd(thd);
     delete thd;
+    dec_thread_count();
   }
   return 1;                                     // Abort handle_one_connection
 }