cache: use size_t for string lengths

Avoid integer truncation on 64-bit systems.

Coverity-id: 13864
Signed-off-by: John Keeping <john@keeping.me.uk>

cache.c

@@ -24,7 +24,7 @@
 
 struct cache_slot {
 	const char *key;
-	int keylen;
+	size_t keylen;
 	int ttl;
 	cache_fill_fn fn;
 	int cache_fd;
@@ -44,7 +44,7 @@ struct cache_slot {
 static int open_slot(struct cache_slot *slot)
 {
 	char *bufz;
-	int bufkeylen = -1;
+	ssize_t bufkeylen = -1;
 
 	slot->cache_fd = open(slot->cache_name, O_RDONLY);
 	if (slot->cache_fd == -1)