DNSSEC: is_nsec3_enabled() check for nonzero algorithm, not salt length

refs #4

src/libknot/dnssec/zone-nsec.c

@@ -827,7 +827,7 @@ bool is_nsec3_enabled(const knot_zone_contents_t *zone)
 		return false;
 	}
 
-	return zone->nsec3_params.salt_length > 0;
+	return zone->nsec3_params.algorithm != 0;
 }
 
 /* - helper functions ------------------------------------------------------ */