Fix zero-object version-2 packs

A pack-file can get created without any objects in it (to transfer "no
data" - which can happen if you use a reference git repo, for example,
or just otherwise just end up transferring only branch head information
and already have all the objects themselves).

And while we probably should never create an index for such a pack, if we
do (and we do), the index file size sanity checking was incorrect.

This fixes it.
Reported-and-tested-by: NJocke Tjernlund <tjernlund@tjernlund.se>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -509,7 +509,10 @@ static int check_packed_git_idx(const char *path,  struct packed_git *p)
 		 * for offsets larger than 2^31.
 		 */
 		unsigned long min_size = 8 + 4*256 + nr*(20 + 4 + 4) + 20 + 20;
-		if (idx_size < min_size || idx_size > min_size + (nr - 1)*8) {
+		unsigned long max_size = min_size;
+		if (nr)
+			max_size += (nr - 1)*8;
+		if (idx_size < min_size || idx_size > max_size) {
 			munmap(idx_map, idx_size);
 			return error("wrong index file size in %s", path);
 		}