DNSSEC: proper parameter checks in zone_sign() in zone events

refs #4

src/libknot/dnssec/zone-events.c

@@ -51,9 +51,9 @@ static void init_forced_policy(knot_dnssec_policy_t *p,
 static int zone_sign(knot_zone_t *zone, knot_changeset_t *out_ch, bool force,
                      knot_update_serial_t soa_up)
 {
-	if (zone == NULL || zone->contents == NULL) {
-		return KNOT_EINVAL;
-	}
+	assert(zone);
+	assert(zone->contents);
+	assert(out_ch);
 
 	dbg_dnssec_verb("Changeset emtpy before generating NSEC chain: %d\n",
 	        knot_changeset_is_empty(out_ch));