PHP-1269: Exception if MongoBinData::UUID_RFC4122 has invalid size

https://jira.mongodb.org/browse/PHP-1269
I'm not sure why `long` was previously being used for this variable. Elsewhere, string lengths are always `int`.

Having this as long actually caused issues with the `PHP_MONGO_BIN_UUID_RFC4122_SIZE` comparison below. Perhaps because zpp only assigns an integer and left the remaining bytes unallocated?