[Code Fix] Early out of bounced messages

[e6654321]

9 - Unchecked bounced messages can still go through the sign check process

[nns2009]

But bounced messages are already ignored by:

if ((op != prefix::extension_action) & (op != prefix::signed_internal)) {
    return (); ;; just receive Toncoins
}

;; bounced messages has 0xffffff prefix and skipped by op check

Between, your suggested code doesn't actually do anything, because original comment has a mistake, and it should be 0xffffffff (two more 'f's)

[tolya-yanot]

@e6654321 incorrect, there is already a check for valid op;

@nns2009 thanks for finding the mistake in the comment