fetch-pack: clear alternate shallow in one more place

The previous one did not clear the variable in one codepath,
but we should aim to be complete.
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
[jc: made a reroll into incremental, as the previous one already is
 in the next branch]
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -1272,6 +1272,8 @@ static void receive_shallow_info(struct fetch_pack_args *args,
 		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file,
 					NULL);
 		args->deepen = 1;
+	} else {
+		alternate_shallow_file = NULL;
 	}
 }