reset the streams' cur_dts values.  This can lead to a fatal "error,
non monotone timestamps ..." message later, because the out-of-date
cur_dts values are used to compute some packet's dts.

Fix this by calling av_read_frame_flush() and eliminate code
duplication in the process.

The additional hunk gives more detailed error messages.

patch by Wolfram Gloger, wmglo dent.med.uni-muenchen de


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@8465 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b