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 message later, because the out-of-date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

Originally committed as revision 8465 to svn://svn.ffmpeg.org/ffmpeg/trunk