pack-redundant: avoid accessing uninitialized variables

The `l` variable holds a list of packs, but its `next` attribute seems
not to be initialized before inserting into the list. Let's ensure that
`l` is initialized properly.

Pointed out by Coverity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/pack-redundant.c

@@ -536,7 +536,7 @@ static void scan_alt_odb_packs(void)
 
 static struct pack_list * add_pack(struct packed_git *p)
 {
-	struct pack_list l;
+	struct pack_list l = { NULL };
 	size_t off = 0, step;
 	const unsigned char *base;