channel.c: Address stack overflow in does_id_conflict()

does_id_conflict() was passing a pointer to an int to a callback
that expected a pointer to a size_t.

Found by the Address Sanitizer.

Change-Id: I0ff542067eef63a14a60301654d65d34fe2ad503

main/channel.c

@@ -751,7 +751,7 @@ static int ast_channel_by_uniqueid_cb(void *obj, void *arg, void *data, int flag
 static int does_id_conflict(const char *uniqueid)
 {
 	struct ast_channel *conflict;
-	int length = 0;
+	size_t length = 0;
 
 	if (ast_strlen_zero(uniqueid)) {
 		return 0;