io: don't leak memory on clean shutdown.

Free the internal "always" array like we free "fds".
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/io/poll.c

@@ -137,6 +137,12 @@ static void remove_from_always(const struct io_plan *plan)
 	if (pos != num_always-1)
 		always[pos] = always[num_always-1];
 	num_always--;
+
+	/* Only free if no fds left either. */
+	if (num_always == 0 && max_fds == 0) {
+		tal_free(always);
+		max_always = 0;
+	}
 }
 
 bool backend_new_always(struct io_plan *plan)