MDEV-20753: Sequence with limit 0 crashes server

Do not try to push down conditions to engine if query was resolved without tables (and so the engine).

sql/sql_select.cc

@@ -2795,7 +2795,7 @@ bool JOIN::make_aggr_tables_info()
     distinct in the engine, so we do this for all queries, not only
     GROUP BY queries.
   */
-  if (tables_list && !procedure)
+  if (tables_list && top_join_tab_count && !procedure)
   {
     /*
       At the moment we only support push down for queries where

storage/sequence/mysql-test/sequence/group_by.result

@@ -97,3 +97,9 @@ count(NULL)
 select count(NULL) from seq_1_to_3;
 count(NULL)
 0
+#
+# MDEV-20753: Sequence with limit 0 crashes server
+#
+select count(NULL) from seq_1_to_3 limit 0;
+count(NULL)
+# End of 10.3 tests

storage/sequence/mysql-test/sequence/group_by.test

@@ -49,3 +49,10 @@ explain select count(*) from seq_1_to_15_step_2 group by mod(seq,2);
 create temporary table t1 select * from seq_1_to_3;
 select count(NULL) from t1;
 select count(NULL) from seq_1_to_3;
+
+--echo #
+--echo # MDEV-20753: Sequence with limit 0 crashes server
+--echo #
+select count(NULL) from seq_1_to_3 limit 0;
+
+--echo # End of 10.3 tests