combine-diff: do not send NULL to printf
combine-diff: do not send NULL to printf

When we run combined diff from working tree (diff-files --cc),
we sent NULL to printf that is returned by find_unique_abbrev().

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

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