Bug #31253: crash comparing datetime to double

convert(<invalid time>, datetime) in WHERE caused crash as function
returned (void*)NULL, but did not flag SQL NULL. It does now.

mysql-test/r/type_datetime.result

@@ -427,3 +427,12 @@ f1
 Warnings:
 Warning	1292	Incorrect datetime value: '2007010100000' for column 'f1' at row 1
 drop table t1;
+create table t1 (f1 time);
+insert into t1 set f1 = '45:44:44';
+insert into t1 set f1 = '15:44:44';
+select * from t1 where (convert(f1,datetime)) != 1;
+f1
+15:44:44
+Warnings:
+Warning	1292	Truncated incorrect datetime value: '0000-00-00 45:44:44'
+drop table t1;

mysql-test/t/type_datetime.test

@@ -282,3 +282,13 @@ select * from t1 where f1 between 20020101 and 20070101000000;
 select * from t1 where f1 between 2002010 and 20070101000000;
 select * from t1 where f1 between 20020101 and 2007010100000;
 drop table t1;
+
+#
+# Bug #31253: crash comparing datetime to double
+# Should return 1st row only. Crashes if NULL propagation fails.
+#
+create table t1 (f1 time);
+insert into t1 set f1 = '45:44:44';
+insert into t1 set f1 = '15:44:44';
+select * from t1 where (convert(f1,datetime)) != 1;
+drop table t1;

sql/item.cc

@@ -274,6 +274,7 @@ my_decimal *Item::val_decimal_from_date(my_decimal *decimal_value)
   if (get_date(&ltime, TIME_FUZZY_DATE))
   {
     my_decimal_set_zero(decimal_value);
+    null_value= 1;                               // set NULL, stop processing
     return 0;
   }
   return date2my_decimal(&ltime, decimal_value);