Fixed crash if max_connections is exceeded. BUG#8996

sql/mysqld.cc:
  mark the thread as killed in close_connection
sql/protocol.cc:
  don't bother remembering warnings if the thread is dying

sql/mysqld.cc

@@ -1432,6 +1432,7 @@ void close_connection(THD *thd, uint errcode, bool lock)
 		      errcode ? ER(errcode) : ""));
   if (lock)
     (void) pthread_mutex_lock(&LOCK_thread_count);
+  thd->killed=1;
   if ((vio=thd->net.vio) != 0)
   {
     if (errcode)

sql/protocol.cc

@@ -122,6 +122,7 @@ void send_error(THD *thd, uint sql_errno, const char *err)
   }
   VOID(net_write_command(net,(uchar) 255, "", 0, (char*) err,length));
 #endif  /* EMBEDDED_LIBRARY*/
+  if (!thd->killed)
     push_warning(thd, MYSQL_ERROR::WARN_LEVEL_ERROR, sql_errno,
                  orig_err ? orig_err : ER(sql_errno));
   thd->is_fatal_error=0;			// Error message is given