vf_vapoursynth: add debug message when returning error from GetFrame

Some filters still (or will) behave badly on these errors, so explicitly
log when it happens.
demux_mkv: fix undefined behavior

With some files, the extradata variable can remain uninitialized, but
will be used for memory access.

CC: @mpv-player/stable (with high priority)
audio/out: add "auto" pseudo-device

Also, don't set an empty string for the fallback device if an AO doesn't
list any devices.
ao_openal: implement device listing
ao_portaudio: implement device listing
ao_dsound: implement device listing
ao_wasapi: implement device listing