don't start purge job if podcast-purge-age-days not set

Co-authored-by: Alex McGrath <amk@amk.ie>

cmd/gonic/gonic.go

@@ -301,6 +301,7 @@ func main() {
 		return nil
 	}, noCleanup)
 
+	if *confPodcastPurgeAgeDays > 0 {
 		g.Add(func() error {
 			log.Printf("starting job 'podcast purger'\n")
 			ticker := time.NewTicker(24 * time.Hour)
@@ -311,6 +312,7 @@ func main() {
 			}
 			return nil
 		}, noCleanup)
+	}
 
 	if *confScanIntervalMins > 0 {
 		g.Add(func() error {