core: make more robust against missing libavcodec AVCodecDescriptor entries

Missing entries cause avcodec_descriptor_get() to return NULL, and in
turn mp_codec_from_av_codec_id() will return NULL. This shouldn't
happen, and avcodec_descriptor_get() returning NULL for a valid codec is
clearly a bug.

But make it more robust anyway, and use the decoder's name if this
happens, because I doubt maintainance of the AVCodecDescriptor table
in ffmpeg/Libav will always be perfect and reliable.