libcvc: handle missing active port in mixer control

[glira]

Summary

PipeWire 1.5.84+ can change a stream's port list while BlueZ switches a Bluetooth headset profile (pipewire#5053). gvc_mixer_stream_get_port() was updated to return NULL when the active port is gone (the libgnome-volume-control change that closed #267), but three callers still dereference that pointer:

• gvc_mixer_control_lookup_device_from_stream
• gvc_mixer_control_change_output
• gvc_mixer_control_change_input

gvc_mixer_control_update_sink and update_source already test for NULL. These three do not, so the old g_assert_not_reached() abort becomes a NULL dereference.

How to reproduce

1. Cinnamon on Ubuntu 26.04 (libcvc 6.4.2-1, or current master with the assert already removed), PipeWire 1.6.2, WirePlumber 0.5.13.
2. Connect a Bluetooth headset (A2DP).

Both cinnamon and csd-media-keys die at the same time:

csd-media-keys: segfault at 0 in libcvc.so.0.0.0
cinnamon:       segfault at 0 in libcvc.so.0.0.0

On 6.4.2, before the assert was removed, the same event aborts instead:

Cvc:ERROR:../libcvc/gvc-mixer-stream.c:541:gvc_mixer_stream_get_port: code should not be reached

The desktop then tries to fall back to Metacity (cinnamon-launcher), which is not installed on Ubuntu, so the session looks like X died.

Fix

Treat a missing active port like a port change: skip the lookup when port is NULL, and call gvc_mixer_stream_change_port() when active_port is NULL.

Verified locally: three Bluetooth disconnect/connect cycles after the check, same cinnamon PID, no new segfault.

Fixes the remaining crash from #267.