Fix for bug #23653: Crash if last_day('0000-00-00')

As get_arg0_date() in the Item_func_last_day::get_date() returns 
0000-00-00 date sometimes, we have to check ltime->month for 0 after the call.

mysql-test/r/func_time.result

@@ -815,4 +815,7 @@ union
 (select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%k') As H);
 H
 5
+select last_day('0000-00-00');
+last_day('0000-00-00')
+NULL
 End of 4.1 tests

mysql-test/t/func_time.test

@@ -446,4 +446,10 @@ union
 union
 (select time_format(timediff(now(), DATE_SUB(now(),INTERVAL 5 HOUR)),'%k') As H);
 
+#
+# Bug #23653: crash if last_day('0000-00-00')
+#
+
+select last_day('0000-00-00');
+
 --echo End of 4.1 tests

sql/item_timefunc.cc

@@ -3058,7 +3058,8 @@ String *Item_func_str_to_date::val_str(String *str)
 
 bool Item_func_last_day::get_date(TIME *ltime, uint fuzzy_date)
 {
-  if (get_arg0_date(ltime, fuzzy_date & ~TIME_FUZZY_DATE))
+  if (get_arg0_date(ltime, fuzzy_date & ~TIME_FUZZY_DATE) ||
+      (ltime->month == 0))
     return 1;
   uint month_idx= ltime->month-1;
   ltime->day= days_in_month[month_idx];