MDEV-10365 - Race condition in error handling of INSERT DELAYED

Shared variables of Delayed_insert may be updated without mutex protection
when delayed insert thread gets an error.

Re-acquire mutex earlier, so that shared variables are protected.

sql/sql_insert.cc

@@ -3238,6 +3238,7 @@ bool Delayed_insert::handle_inserts(void)
   max_rows= 0;                                  // For DBUG output
 #endif
   /* Remove all not used rows */
+  mysql_mutex_lock(&mutex);
   while ((row=rows.get()))
   {
     if (table->s->blob_fields)
@@ -3254,7 +3255,6 @@ bool Delayed_insert::handle_inserts(void)
   }
   DBUG_PRINT("error", ("dropped %lu rows after an error", max_rows));
   thread_safe_increment(delayed_insert_errors, &LOCK_delayed_status);
-  mysql_mutex_lock(&mutex);
   DBUG_RETURN(1);
 }
 #endif /* EMBEDDED_LIBRARY */