pack: dereference cached pack entry on error

pack: dereference cached pack entry on error

When trying to uncompress deltas in a packfile's delta chain, we try to
add object bases to the packfile cache, subsequently decrementing its
reference count if it has been added successfully. This may lead to a
mismatched reference count in the case where we exit the loop early due
to an encountered error.

Fix the issue by decrementing the reference count in error cleanup.