PHP-737: Remove invalid argument casting

Possibly a copy-paste error. This cast was necessary in php_mongo_ctor() since error_message was a (char *), but it's alreay of the appropriate type here.
Be more conservative with malloc() for RP tags error message

This is consistent with error message allocation in mongo_store_option().
Merged Pull Request #387

# By Jeremy Mikola
* pull-request/387:
  Be more conservative with malloc() for RP tags error message
  PHP-737: Remove invalid argument casting