Ensure the pack index is opened before access

In this particular location of fsck the index should have already
been opened by verify_pack, which is called just before we get
here and loop through the object names.  However, just in case a
future version of that function does not use the index file we'll
double-check its open before we access the num_objects field.

Better safe now than sorry later.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-fsck.c

@@ -668,7 +668,10 @@ int cmd_fsck(int argc, char **argv, const char *prefix)
 			verify_pack(p, 0);
 
 		for (p = packed_git; p; p = p->next) {
-			uint32_t i, num = p->num_objects;
+			uint32_t i, num;
+			if (open_pack_index(p))
+				continue;
+			num = p->num_objects;
 			for (i = 0; i < num; i++)
 				fsck_sha1(nth_packed_object_sha1(p, i));
 		}