Look for the validator being canceled in resume_answer

This should speed up shutting down a validation that has
been cancelled.

lib/dns/validator.c

@@ -1794,6 +1794,11 @@ static void
 resume_answer(void *arg) {
 	dns_validator_t *val = arg;
 
+	if (CANCELED(val) || CANCELING(val)) {
+		validate_async_done(val, ISC_R_CANCELED);
+		return;
+	}
+
 	val->resume = true;
 	validate_answer_iter_start(val);
 }