fix 'seektest' again

backport  r19270 by rbultje:

Remove any reference to ASFContext.packet_size and replace it with
AVFormatContext.packet_size. See "[PATCH] asf*.c/h: use
AVFormatContext->packet_size instead of own copy" thread on ML.

and r19361 by reimar:

Check for packet_length 0, it is already treated as invalid by the padding check,
but that resulted in a confusing/wrong error message.



Originally committed as revision 22147 to svn://svn.ffmpeg.org/ffmpeg/branches/0.5