parse_commit: don't fail, if object is NULL
parse_commit: don't fail, if object is NULL

Some codepaths (eg. builtin-rev-parse -> get_merge_bases -> parse_commit)
can pass NULL.

Signed-off-by: Martin Koegler <ea56f0187842ce0af18f0283b5f003697482ffdd@auto.tuwien.ac.at>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git