libnetmap: reset errno in nmreq_register_decode()

The reset is necessary at the beginning of the function, because of
the errno logic in the error path (set errno to EINVAL if not set).
If errno is already set when calling the function, and the function
fails, the previous errno value will be inherited.

(cherry picked from commit ab639bb2873034786cd2ec4d2d9c4489fbf6f424)