avformat/asfdec: call ff_read_frame_flush() in asf_read_pts()

flushing just the asf demuxer but not the cores buffers leads to inconsistencies
and a "random" packet position which later causes an assertion failure

Fixes Ticket2853

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg backport for assettion in wma
Merge pull request #4375 from FernetMenta/wma

fix assert when playing certain wma files