Fix PHP segfault when missing function `Class::table_name`.

If the subclass is missing `Class::table_name` `__call` would be called, in turn calling `__get` which needs `Class::table_name` to work, repeat. Result is infinite recursion and a PHP segmentation fault.
Merge pull request #23 from ext/table_name_segfault

Fix PHP segfault when missing function `Class::table_name`.