Instead of just checking for an empty buffer, which may not be the case if

there is unconsumed data, save the previous size and use it instead. This means
that activity monitoring should work in this (unlikely) event.

Also remove a debugging statement that no longer seems necessary.

tmux.h

@@ -617,6 +617,7 @@ struct input_ctx {
 	u_char		*buf;
 	size_t		 len;
 	size_t		 off;
+	size_t		 was;
 
 	struct grid_cell cell;