posix: added check for port closing

serial_unix.go

@@ -33,6 +33,7 @@ func (port *unixPort) Close() error {
 		return err
 	}
 
+	if port.closeSignal != nil {
 		// Send close signal to all pending reads (if any)
 		port.closeSignal.Write([]byte{0})
 
@@ -44,6 +45,7 @@ func (port *unixPort) Close() error {
 		if err := port.closeSignal.Close(); err != nil {
 			return err
 		}
+	}
 	return nil
 }