Fix issue 317

This is the exact same fix as mentionne din the GC issue,
cf. https://code.google.com/p/libarchive/issues/detail?id=317
Issue 314:  A tar archive containing only a single 'g' record
should be treated as a valid empty tar archive.
(Such archives are generated by 'git archive' from an empty
repository.)

http://thread.gmane.org/gmane.comp.version-control.git/220485
http://code.google.com/p/libarchive/issues/detail?id=314