vaapidecode: fix decoder flush.

vaapidecode: fix decoder flush.

There are situations where gst_video_decoder_flush() is called, and
this subsequently produces a gst_video_decoder_reset() that kills the
currently active GstVideoCodecFrame. This means that it no longer
exists by the time we reach GstVideoDecoder::finish() callback, thus
possibly resulting in a crash if we assumed spare data was still
available for decode (current_frame_size > 0).

Try to honour GstVideoDecoder::reset() behaviour from GStreamer 1.0
that means a flush, thus performing the actual operations there like
calling gst_video_decoder_have_frame() if pending data is available.