BUG#780359: Crash with get_fanout_with_deps in maria-5.3-mwl90

- Handle the case of degenerate joins. When join has impossible WHERE clause,
  a number of its members, including JOIN::map2table, are not initialized. 
  Do not try accessing them, detect degenerate joins early.

mysql-test/r/subselect_sj_nonmerged.result

@@ -81,3 +81,21 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	10	Using join buffer (flat, BNL join)
 drop table t1,t2,t3,t4;
 drop table t0;
+#
+# BUG#780359: Crash with get_fanout_with_deps in maria-5.3-mwl90
+#
+CREATE TABLE t1 (f1 int);
+INSERT INTO t1 VALUES (2),(2);
+CREATE TABLE t2 (f3 int);
+INSERT INTO t2 VALUES (2),(2);
+SELECT *
+FROM t1
+WHERE ( f1 ) IN (
+SELECT t2.f3
+FROM t2
+WHERE t2.f3 = 97
+AND t2.f3 = 50
+GROUP BY 1
+);
+f1
+DROP TABLE t1, t2;

mysql-test/t/subselect_sj_nonmerged.test

@@ -74,3 +74,24 @@ drop table t1,t2,t3,t4;
 
 drop table t0;
 
+--echo #
+--echo # BUG#780359: Crash with get_fanout_with_deps in maria-5.3-mwl90
+--echo #
+CREATE TABLE t1 (f1 int);
+INSERT INTO t1 VALUES (2),(2);
+
+CREATE TABLE t2 (f3 int);
+INSERT INTO t2 VALUES (2),(2);
+
+SELECT *
+FROM t1
+WHERE ( f1 ) IN (
+        SELECT t2.f3
+        FROM t2
+        WHERE t2.f3 = 97
+        AND t2.f3 = 50
+        GROUP BY 1
+);
+
+DROP TABLE t1, t2;
+

sql/item_subselect.cc

@@ -4094,6 +4094,10 @@ JOIN_TAB *next_top_level_tab(JOIN *join, JOIN_TAB *tab);
 
 double get_fanout_with_deps(JOIN *join, table_map tset)
 {
+  /* Handle the case of "Impossible WHERE" */
+  if (join->table_count == 0)
+    return 0.0;
+
   /* First, recursively get all tables we depend on */
   table_map deps_to_check= tset;
   table_map checked_deps= 0;