Fix possible NULL dereference in to_date() builtin

StringVal TimestampFunctions::ToDate(FunctionContext* context,
const TimestampVal& ts_val) {
if (ts_val.is_null) return StringVal::null();
const TimestampValue ts_value = TimestampValue::FromTimestampVal(ts_val);
// Defensively, return NULL if the timestamp does not have a date portion. Some of
// our built-in functions might incorrectly return such a malformed timestamp.
if (!ts_value.HasDate()) return StringVal::null();
StringVal result(context, 10);
result.len = 10;
// Fill in year, month, and day.
IntToChar(result.ptr, ts_value.date().year(), 4);  <-----
IntToChar(result.ptr + 5, ts_value.date().month(), 2);
IntToChar(result.ptr + 8, ts_value.date().day(), 2);
// Fill in dashes.
result.ptr[7] = '-';
result.ptr[4] = '-';
return result;
}