Relax sanity check of number fields in tar header even more.

This fixes reading tar archives created by older versions of
plexus-archiver (used by maven-assembly-plugin) that fill uid and gid
fields with spaces (PLXCOMP-233).

Vendor issue:
https://github.com/libarchive/libarchive/issues/832

Reported by: Antoine Brodin <antoine@freebsd.org>
MFC after: 1 week