Fix a small memory leak in util_errmap

Fix a small memory leak in util_errmap

Calls to gssint_mecherrmap_map_errcode would result in calling
mecherror_copy with a zero-length mech OID, which would result in an
OID with 0 for length and malloc(0) for elements.  On platforms which
return non-null from malloc(0), gssint_mecherrmap_destroy() wouldn't
free the elements pointer.

Avoid calling malloc(0) and don't use the length field to decide
whether to free an elements pointer.