fix of LP bug#998321

The problem is that we can't check null_value field of non-basic constant without the item execution.:

mysql-test/r/func_if.result

@@ -186,3 +186,20 @@ MAX(IFNULL(CAST(c AS UNSIGNED), 0))
 12345678901234567890
 DROP TABLE t1;
 End of 5.0 tests
+#
+# LP bug#998321 Simple query with IF expression causes an
+# assertion abort (see also mysql Bug#12620084)
+#
+SELECT if(0, (SELECT min('hello')), NULL);
+if(0, (SELECT min('hello')), NULL)
+NULL
+SELECT if(1, (SELECT min('hello')), NULL);
+if(1, (SELECT min('hello')), NULL)
+hello
+SELECT if(0, NULL, (SELECT min('hello')));
+if(0, NULL, (SELECT min('hello')))
+hello
+SELECT if(1, NULL, (SELECT min('hello')));
+if(1, NULL, (SELECT min('hello')))
+NULL
+End of 5.2 tests

mysql-test/t/func_if.test

@@ -170,6 +170,19 @@ DROP TABLE t1;
 
 --echo End of 5.0 tests
 
+
+--echo #
+--echo # LP bug#998321 Simple query with IF expression causes an
+--echo # assertion abort (see also mysql Bug#12620084)
+--echo #
+
+SELECT if(0, (SELECT min('hello')), NULL);
+SELECT if(1, (SELECT min('hello')), NULL);
+SELECT if(0, NULL, (SELECT min('hello')));
+SELECT if(1, NULL, (SELECT min('hello')));
+
+--echo End of 5.2 tests
+
 --disable_query_log
 # Restore timezone to default
 set time_zone= @@global.time_zone;

sql/item_cmpfunc.cc

@@ -2659,8 +2659,8 @@ Item_func_if::fix_length_and_dec()
 
   enum Item_result arg1_type=args[1]->result_type();
   enum Item_result arg2_type=args[2]->result_type();
-  bool null1=args[1]->const_item() && args[1]->null_value;
-  bool null2=args[2]->const_item() && args[2]->null_value;
+  bool null1=args[1]->basic_const_item() && args[1]->null_value;
+  bool null2=args[2]->basic_const_item() && args[2]->null_value;
 
   if (null1)
   {