Check for nil listener before printing address (#1946)

* Checking that a server listener is not nil before printing verbose information

* Improved readability of a loop

caddy.go

@@ -518,6 +518,11 @@ func startWithListenerFds(cdyfile Input, inst *Instance, restartFds map[string]r
 		}
 		if !Quiet {
 			for _, srvln := range inst.servers {
+				// only show FD notice if the listener is not nil.
+				// This can happen when only serving UDP or TCP
+				if srvln.listener == nil {
+					continue
+				}
 				if !IsLoopback(srvln.listener.Addr().String()) {
 					checkFdlimit()
 					break