MDEV-20074: Lost connection on update trigger

Instead of checking lex->sql_command which does not corect in case of triggers
mark tables for insert.

sql/sql_lex.h

@@ -4060,6 +4060,8 @@ public:
   }
   bool tvc_finalize();
   bool tvc_finalize_derived();
+
+  void mark_first_table_as_inserting();
 };