fix: check return code in `unpack_callback_int64` by KowalskiThomas · Pull Request #665 · msgpack/msgpack-python

Similar to #665, just a return value check to propagate the error in
case one happens (instead of silently ignoring it).

Note that as opposed to the previous lines, we don't need to
`PyErr_SetString` since `unpack_callback_uint32` calls
`PyLong_FromSize_t` which itself should set whatever Python error is
relevant; we just need to make it clear to the caller that an error
occurred.

Refactor unpack_callback_int64 to simplify PyObject creation.