Make electronicarts demuxer return partial frames, this is the default

Make electronicarts demuxer return partial frames, this is the default
behaviour of av_get_packet and should not be override without good reason.
As a side effect this fixes the memleak described in issue 956.
Also return the exact error code from av_get_packet instead of AVERROR(EIO).

Originally committed as revision 18428 to svn://svn.ffmpeg.org/ffmpeg/trunk