MDEV-5504 Server crashes in String::length on SELECT with MONTHNAME, GROUP BY, ROLLUP

The crash happened because Item_func_monthname was derived from
Item_func_month, so Item_func_monthname::is_null() did not work fine.
Backporting a change from 5.5: Item_func_monthname is now derived from
Item_str_func.

mysql-test/r/func_time.result

@@ -162,7 +162,7 @@ dayname("1962-03-03")	dayname("1962-03-03")+0
 Saturday	5
 select monthname("1972-03-04"),monthname("1972-03-04")+0;
 monthname("1972-03-04")	monthname("1972-03-04")+0
-March	3
+March	0
 select time_format(000000,'%H|%I|%k|%l|%i|%p|%r|%S|%T'),date_format(19980131000000,'%H|%I|%k|%l|%i|%p|%r|%S|%T');
 time_format(000000,'%H|%I|%k|%l|%i|%p|%r|%S|%T')	date_format(19980131000000,'%H|%I|%k|%l|%i|%p|%r|%S|%T')
 00|12|0|12|00|AM|12:00:00 AM|00|00:00:00	00|12|0|12|00|AM|12:00:00 AM|00|00:00:00
@@ -2347,3 +2347,13 @@ EXTRACT(HOUR FROM TIME('1 02:00:00'))	EXTRACT(HOUR FROM TIME('26:00:00'))
 SELECT EXTRACT(DAY FROM TIME('1 02:00:00')), EXTRACT(DAY FROM TIME('26:00:00'));
 EXTRACT(DAY FROM TIME('1 02:00:00'))	EXTRACT(DAY FROM TIME('26:00:00'))
 1	1
+#
+# MDEV-5504 Server crashes in String::length on SELECT with MONTHNAME, GROUP BY, ROLLUP
+#
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+SELECT 1 FROM t1 GROUP BY MONTHNAME(0) WITH ROLLUP;
+1
+1
+1
+DROP TABLE t1;

mysql-test/t/func_time.test

@@ -1417,3 +1417,12 @@ SELECT EXTRACT(HOUR FROM '1 02:00:00'), EXTRACT(HOUR FROM '26:00:00');
 SELECT EXTRACT(HOUR FROM TIME'1 02:00:00'), EXTRACT(HOUR FROM TIME'26:00:00');
 SELECT EXTRACT(HOUR FROM TIME('1 02:00:00')), EXTRACT(HOUR FROM TIME('26:00:00'));
 SELECT EXTRACT(DAY FROM TIME('1 02:00:00')), EXTRACT(DAY FROM TIME('26:00:00'));
+
+
+--echo #
+--echo # MDEV-5504 Server crashes in String::length on SELECT with MONTHNAME, GROUP BY, ROLLUP
+--echo #
+CREATE TABLE t1 (i INT);
+INSERT INTO t1 VALUES (1),(2);
+SELECT 1 FROM t1 GROUP BY MONTHNAME(0) WITH ROLLUP;
+DROP TABLE t1;

sql/item_timefunc.cc

@@ -883,16 +883,13 @@ String* Item_func_monthname::val_str(String* str)
 {
   DBUG_ASSERT(fixed == 1);
   const char *month_name;
-  uint month= (uint) val_int();
   uint err;
+  MYSQL_TIME ltime;
 
-  if (null_value || !month)
-  {
-    null_value=1;
-    return (String*) 0;
-  }
-  null_value=0;
-  month_name= locale->month_names->type_names[month-1];
+  if ((null_value= (get_arg0_date(&ltime, 0) || !ltime.month)))
+    return (String *) 0;
+
+  month_name= locale->month_names->type_names[ltime.month - 1];
   str->copy(month_name, (uint) strlen(month_name), &my_charset_utf8_bin,
 	    collation.collation, &err);
   return str;

sql/item_timefunc.h

@@ -134,14 +134,13 @@ public:
 };
 
 
-class Item_func_monthname :public Item_func_month
+class Item_func_monthname :public Item_str_func
 {
   MY_LOCALE *locale;
 public:
-  Item_func_monthname(Item *a) :Item_func_month(a) {}
+  Item_func_monthname(Item *a) :Item_str_func(a) {}
   const char *func_name() const { return "monthname"; }
   String *val_str(String *str);
-  enum Item_result result_type () const { return STRING_RESULT; }
   void fix_length_and_dec();
   bool check_partition_func_processor(uchar *int_arg) {return TRUE;}
   bool check_vcol_func_processor(uchar *int_arg) {return FALSE;}