MDEV-714 LP:1020645 - crash (sig 11) with union query

Don't ignore the error from st_select_lex_unit::optimize()

mysql-test/r/union_crash-714.result

+create table t1 (i tinyint);
+set debug='+d,bug11747970_raise_error';
+insert into t1 (i) select i from t1 union select i from t1;
+ERROR 70100: Query execution was interrupted
+drop table t1;

mysql-test/t/union_crash-714.test

+#
+# MDEV-714 LP:1020645 - crash (sig 11) with union query
+#
+--source include/have_debug.inc
+create table t1 (i tinyint);
+set debug='+d,bug11747970_raise_error';
+--error ER_QUERY_INTERRUPTED
+insert into t1 (i) select i from t1 union select i from t1;
+drop table t1;

sql/sql_union.cc

@@ -629,7 +629,8 @@ bool st_select_lex_unit::exec()
   if (!(uncacheable & ~UNCACHEABLE_EXPLAIN) && item)
     item->make_const();
   
-  saved_error= optimize();
+  if ((saved_error= optimize()))
+    DBUG_RETURN(saved_error);
 
   if (uncacheable || !item || !item->assigned() || describe)
   {