eina: fix behaviour break of eina_error_msg_get()

eina: fix behaviour break of eina_error_msg_get()

eina_error_msg_get() must return NULL if an incorrect error is provided.
The XSI strerror_r() returns EINVAL when an invalid error is passed to
it, so we can end the function here. If we kept on, we would have tested
against the 'unknown_prefix' ("Unknown error ") which is implementation
defined, and registered a new error when the invalid error message
didn't match the 'unknown_prefix'. This new error message would have
been returned, which is not what we expected.

This case arised on Mac OS X where the 'unkwown prefix' is
"Unknown error: " instead of "Unknown error ".

It fixes eina test suite on Mac OS X.