Fix use after free in transaction code

If we set an instruction as ready twice, it decreases the transaction's
num_waiting a second time and applies the transaction earlier than it
should. This no doubt has undesired effects, probably resulting in a use
after free.

Hopefully fixes the first part of #2207.

sway/desktop/transaction.c

@@ -377,9 +377,11 @@ static void set_instructions_ready(struct sway_view *view, int index) {
 	for (int i = 0; i <= index; ++i) {
 		struct sway_transaction_instruction *instruction =
 			view->swayc->instructions->items[i];
+		if (!instruction->ready) {
 			set_instruction_ready(instruction);
 		}
 	}
+}
 
 void transaction_notify_view_ready(struct sway_view *view, uint32_t serial) {
 	for (int i = 0; i < view->swayc->instructions->length; ++i) {