Bug#20129: ALTER TABLE ... REPAIR PARTITION ... complains that

partition is corrupt

Post push fix

an DBUG_ASSERT broke the embedded server, fixed by initializing
it in the embedded version of Protocol_text::prepare_for_resend

libmysqld/lib_sql.cc

@@ -1103,6 +1103,9 @@ void Protocol_text::prepare_for_resend()
   data->embedded_info->prev_ptr= &cur->next;
   next_field=cur->data;
   next_mysql_field= data->embedded_info->fields_list;
+#ifndef DBUG_OFF
+  field_pos= 0;
+#endif
 
   DBUG_VOID_RETURN;
 }