pack-objects: return early when computing write order fails

The function `compute_write_order` may return a `NULL`-pointer
when an error occurs. In such cases we jump to the `done`-label
where we try to clean up allocated memory. Unfortunately we try
to deallocate the `write_order` array, though, which may be NULL
here.

Fix this error by returning early instead of jumping to the
`done` label. There is no data to be cleaned up anyway.

src/pack-objects.c

@@ -629,10 +629,8 @@ static int write_pack(git_packbuilder *pb,
 	int error = 0;
 
 	write_order = compute_write_order(pb);
-	if (write_order == NULL) {
-		error = -1;
-		goto done;
-	}
+	if (write_order == NULL)
+		return -1;
 
 	/* Write pack header */
 	ph.hdr_signature = htonl(PACK_SIGNATURE);