fix hang in git fetch if pointed at a 0 length bundle

git-repo if interupted at the exact wrong time will generate zero
length bundles- literal empty files.  git-repo is wrong here, but
git fetch shouldn't effectively spin loop if pointed at a zero
length bundle.

Signed-off-by: Brian Harring <ferringb@chromium.org>
Helped-by: Johannes Sixt
Helped-by: Nguyen Thai Ngoc Duy
Signed-off-by: Junio C Hamano <gitster@pobox.com>