fix for bug#15828

problem was not checking 2nd parameter of str_to_date against NULL

mysql-test/r/date_formats.result

@@ -464,3 +464,9 @@ d1	d2
 02	February
 01	January
 drop table t1;
+select str_to_date( 1, NULL );
+str_to_date( 1, NULL )
+NULL
+select str_to_date( NULL, 1 );
+str_to_date( NULL, 1 )
+NULL

mysql-test/t/date_formats.test

@@ -268,4 +268,11 @@ insert into t1 (f1) values ("2005-01-01");
 insert into t1 (f1) values ("2005-02-01");
 select date_format(f1, "%m") as d1, date_format(f1, "%M") as d2 from t1 order by date_format(f1, "%M");
 drop table t1;
+
+#
+# Bug #15828
+#
+select str_to_date( 1, NULL );
+select str_to_date( NULL, 1 );
+
 # End of 4.1 tests

sql/item_timefunc.cc

@@ -2750,7 +2750,7 @@ void Item_func_str_to_date::fix_length_and_dec()
   cached_field_type= MYSQL_TYPE_STRING;
   max_length= MAX_DATETIME_FULL_WIDTH*MY_CHARSET_BIN_MB_MAXLEN;
   cached_timestamp_type= MYSQL_TIMESTAMP_NONE;
-  if ((const_item= args[1]->const_item()))
+  if ((const_item= args[1]->const_item()) && !args[1]->null_value)
   {
     format= args[1]->val_str(&format_str);
     cached_format_type= get_date_time_result_type(format->ptr(),