lib/repo_pkgdeps.c: check correctly for errno after our call, not after free(3).

The issue was that xbps_pkgdb_get_pkg() did not find any package,
and the code was free(3)ing heap allocated memory before checking for
errno. I suspect that free(3) has touched errno and this errno value
has been propagated to the next code.

Found after a bit of testing on repo.voidlinux.eu.