i.e. the four bytes 00 00 01 0a.
When decode_nal() decodes the end of sequence NAL unit, it returns with
dst_length == 0. The original code leads to a return -1 which discards
the current properly decoded frame.
patch by Reinhard Nissl, rnissl gmx de

Originally committed as revision 8688 to svn://svn.ffmpeg.org/ffmpeg/trunk