fix(icom): a disconnected radio says so, not 'use the USB sound card'

The TX-audio entry points type-assert the transport; a nil port failed the
assertion and produced the sound-card message, sending an operator whose
radio was simply off — or on a new DHCP address — hunting through audio
devices.

internal/cat/icomserial.go

@@ -2072,6 +2072,12 @@ func (b *IcomSerial) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan str
 	// from (IcomDo); at worst a reconnect leaves this one connection stale, and
 	// a stale transport fails fast on its closed done channel.
 	port := b.port
+	// A nil port fails the assertions below too — and used to fail them with
+	// the sound-card message, sending an operator whose radio was simply OFF
+	// hunting through their audio devices.
+	if port == nil {
+		return fmt.Errorf("not connected to the radio — check the CAT link")
+	}
 	type txPlayer interface {
 		PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan struct{}) error
 	}
@@ -2085,6 +2091,9 @@ func (b *IcomSerial) PlayTXAudio(pcm []byte, rate, ch, bits int, stop <-chan str
 // rig is reached over one.
 func (b *IcomSerial) TXAudioSender() (func([]byte) error, error) {
 	port := b.port
+	if port == nil {
+		return nil, fmt.Errorf("not connected to the radio — check the CAT link")
+	}
 	type sender interface {
 		TXAudioSender() (func([]byte) error, error)
 	}