mark packages not found with pacsift as unavailable

proto_package.go

@@ -453,10 +453,13 @@ func (p *ProtoPackage) isAvailable(h *alpm.Handle) bool {
 		var res []byte
 		res, err = cmd.CombinedOutput()
 		log.Debug(string(res))
-		if err != nil || len(res) == 0 {
+		if err != nil {
 			log.Warningf("error getting packages from pacsift for %s: %v", p.Pkgbase, err)
 			buildManager.alpmMutex.Unlock()
 			return false
+		} else if len(res) == 0 {
+			buildManager.alpmMutex.Unlock()
+			return false
 		}
 
 		if len(strings.Split(strings.TrimSpace(string(res)), "\n")) > 0 {