cache: don't use an integer as a NULL pointer

Signed-off-by: John Keeping <john@keeping.me.uk>

cache.c

@@ -411,7 +411,7 @@ int cache_ls(const char *path)
 	DIR *dir;
 	struct dirent *ent;
 	int err = 0;
-	struct cache_slot slot = { 0 };
+	struct cache_slot slot = { NULL };
 	struct strbuf fullname = STRBUF_INIT;
 	size_t prefixlen;