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: brian m. carlson <sandals@crustytoothpaste.net>
[jc: made a reroll into incremental, as the previous one already is
 in the next branch]
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/fetch-pack.c b/fetch-pack.c
index 2d76287..a92621a 100644
--- a/fetch-pack.c
+++ b/fetch-pack.c
@@ -1272,6 +1272,8 @@
 		setup_alternate_shallow(&shallow_lock, &alternate_shallow_file,
 					NULL);
 		args->deepen = 1;
+	} else {
+		alternate_shallow_file = NULL;
 	}
 }