Fixed the bug mdev-12554.

When the WHERE condition is always FALSE then JOIN::join_tab is NULL.

mysql-test/r/cte_recursive.result

@@ -2759,3 +2759,15 @@ set standard_compliant_cte=default;
 select @@standard_compliant_cte;
 @@standard_compliant_cte
 1
+#
+# mdev-12554: impossible where in recursive select 
+#             
+CREATE TABLE t1 (i int);
+INSERT INTO t1 VALUES (1),(2);
+WITH RECURSIVE
+cte(f) AS ( SELECT i FROM t1 UNION SELECT f FROM t1, cte WHERE 1=0 )
+SELECT * FROM cte;
+f
+1
+2
+DROP TABLE t1;

mysql-test/t/cte_recursive.test

@@ -1834,3 +1834,15 @@ drop table objects, modules, module_arguments, module_results;
 set standard_compliant_cte=default;
 select @@standard_compliant_cte;
 
+--echo #
+--echo # mdev-12554: impossible where in recursive select 
+--echo #             
+
+CREATE TABLE t1 (i int);
+INSERT INTO t1 VALUES (1),(2);
+
+WITH RECURSIVE
+cte(f) AS ( SELECT i FROM t1 UNION SELECT f FROM t1, cte WHERE 1=0 )
+SELECT * FROM cte;
+
+DROP TABLE t1;

sql/sql_union.cc

@@ -1261,7 +1261,7 @@ bool st_select_lex_unit::exec_recursive()
                                                  !is_unrestricted);
     if (!with_element->rec_result->first_rec_table_to_update)
       with_element->rec_result->first_rec_table_to_update= rec_table;
-    if (with_element->level == 1)
+    if (with_element->level == 1 && rec_table->reginfo.join_tab)
       rec_table->reginfo.join_tab->preread_init_done= true;  
   }
   for (Item_subselect *sq= with_element->sq_with_rec_ref.first;