MDEV-5106: Server crashes in Explain_union::print_explain on ER_TOO_BIG_SELECT

- Don't save UNION's EXPLAIN data if optimization failed with 
  an error. We could end up saving incomplete plan, which will 
  cause a crash when we attempt to print it.

sql/sql_explain.cc

@@ -157,6 +157,10 @@ bool print_explain_query(LEX *lex, THD *thd, String *str)
 }
 
 
+/* 
+  Return tabular EXPLAIN output as a text string
+*/
+
 bool Explain_query::print_explain_str(THD *thd, String *out_str)
 {
   List<Item> fields;