Merge branch 'rs/fetch-pack-invalid-lockfile'

"fetch-pack" could pass NULL pointer to unlink(2) when it sees an
invalid filename; the error checking has been tightened to make
this impossible.

* rs/fetch-pack-invalid-lockfile:
  fetch-pack: disregard invalid pack lockfiles

fetch-pack.c

@@ -918,8 +918,9 @@ static int get_pack(struct fetch_pack_args *args,
 	if (start_command(&cmd))
 		die(_("fetch-pack: unable to fork off %s"), cmd_name);
 	if (do_keep && pack_lockfiles) {
-		string_list_append_nodup(pack_lockfiles,
-					 index_pack_lockfile(cmd.out));
+		char *pack_lockfile = index_pack_lockfile(cmd.out);
+		if (pack_lockfile)
+			string_list_append_nodup(pack_lockfiles, pack_lockfile);
 		close(cmd.out);
 	}