mixer-control: Check for invalid input

mixer-control: Check for invalid input

With pipewire 1.5.84 there can be a zombie default-source as the
current profile can't have a stream, see

https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/5053#note_3256202

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Origin: https://gitlab.gnome.org/GNOME/libgnome-volume-control/-/merge_requests/31
Bug: https://gitlab.gnome.org/GNOME/libgnome-volume-control/-/issues/34
Bug-Debian: https://bugs.debian.org/1126514
Applied-upstream: gnome-shell 50, commit:798230599950607ec285daecd070f5bcb28580e4

Gbp-Pq: Topic libgvc
Gbp-Pq: Name mixer-control-Check-for-invalid-input.patch