pack-objects: remove unnecessary NULL check

pack-objects: remove unnecessary NULL check

If done_pbase_paths is NULL then done_pbase_paths_num must be zero and
done_pbase_path_pos() returns -1 without accessing the array, so the
check is not necessary.

If the invariant was violated then the check would make sure we keep
on going and allocate the necessary amount of memory in the next
ALLOC_GROW call.  That sounds nice, but all array entries except for
one would contain garbage data.

If the invariant was violated without the check we'd get a segfault in
done_pbase_path_pos(), i.e. an observable crash, alerting us of the
presence of a bug.

Currently there is no such bug: Only the functions check_pbase_path()
and cleanup_preferred_base() change pointer and counter, and both make
sure to keep them in sync.  Get rid of the check anyway to allow us to
see if later changes introduce such a defect, and to simplify the code.

Detected by Coverity Scan.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>