pack-objects: do not get distracted by broken symrefs

pack-objects: do not get distracted by broken symrefs

It is quite possible for, say, a remote HEAD to become broken, e.g.
when the default branch was renamed.

We should still be able to pack our objects when such a thing happens;
simply ignore broken symrefs (because they cannot matter for the packing
process anyway).

This fixes https://github.com/git-for-windows/git/issues/423

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>