mixer-stream: handle portless streams during profile transitions

[PhilBrk8]

Summary

• Guard against NULL ports in gvc_mixer_stream_get_port()
• Log a debug message and return NULL instead of asserting
• Prevents GNOME session crashes during Bluetooth profile switches

Background

When WirePlumber switches Bluetooth profiles (A2DP ↔ HFP),
the stream can be in a transient state where stream->priv->ports is NULL.
GVC currently asserts on this, which crashes gsd-media-keys and gnome-shell.

Fix

Make gvc_mixer_stream_get_port() tolerant of NULL ports and
return NULL with a debug log instead of aborting.

Cross‑reference

Companion mitigation in WirePlumber PR:
PipeWire/wireplumber#16
This defers profile switching until EnumRoute data is available, reducing the likelihood of the portless transient state.

Test plan

• Not run: libgnome-volume-control is intended as a subproject.
• Validate via parent project (e.g., gnome-settings-daemon) and by switching Bluetooth profiles in GNOME Settings.

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