BUG#723822: Crash in get_constant_key_infix with EXISTS ( SELECT .. DISTINCT )

- Make get_constant_key_infix() take into account that there may be SEL_TREEs with
  type=SEL_ARG::MAYBE_KEY, which it cannot process, because they are not real ranges
  but rather indications that we might have been able to construct a range if we had
  values for some other tables' fields. 
  (check_quick_select() already has such check)

mysql-test/r/subselect4.result

@@ -689,3 +689,21 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 2	DEPENDENT SUBQUERY	X	ALL	NULL	NULL	NULL	NULL	10	
 2	DEPENDENT SUBQUERY	B	ALL	a,b	NULL	NULL	NULL	1000	Range checked for each record (index map: 0x3)
 drop table t1, t2;
+#
+# BUG#723822: Crash in get_constant_key_infix with EXISTS ( SELECT .. DISTINCT )
+#
+CREATE TABLE t1 ( f1 int(11), f3 varchar(1)) ;
+INSERT INTO t1 VALUES ('8','c'),('5','f');
+ALTER TABLE t1 ADD KEY (f3,f1);
+CREATE TABLE t2 ( f4 varchar(1)) ;
+INSERT INTO t2 VALUES ('f'),('d');
+SELECT * FROM t2
+WHERE EXISTS (
+SELECT DISTINCT f3
+FROM t1
+WHERE f3 <= t2.f4
+);
+f4
+f
+d
+drop table t1,t2;

mysql-test/t/subselect4.test

@@ -619,3 +619,24 @@ select a,
 from t1 A;  
 drop table t1, t2;
 
+
+--echo #
+--echo # BUG#723822: Crash in get_constant_key_infix with EXISTS ( SELECT .. DISTINCT )
+--echo #
+CREATE TABLE t1 ( f1 int(11), f3 varchar(1)) ;
+INSERT INTO t1 VALUES ('8','c'),('5','f');
+
+ALTER TABLE t1 ADD KEY (f3,f1);
+
+CREATE TABLE t2 ( f4 varchar(1)) ;
+INSERT INTO t2 VALUES ('f'),('d');
+
+SELECT * FROM t2
+WHERE EXISTS (
+        SELECT DISTINCT f3
+        FROM t1
+        WHERE f3 <= t2.f4
+);
+
+drop table t1,t2;
+

sql/opt_range.cc

@@ -11857,7 +11857,8 @@ get_constant_key_infix(KEY *index_info, SEL_ARG *index_range_tree,
       Find the range tree for the current keypart. We assume that
       index_range_tree points to the leftmost keypart in the index.
     */
-    for (cur_range= index_range_tree; cur_range;
+    for (cur_range= index_range_tree; 
+         cur_range && cur_range->type == SEL_ARG::KEY_RANGE;
          cur_range= cur_range->next_key_part)
     {
       if (cur_range->field->eq(cur_part->field))