wtvfile_read_packet did not abide by the requirements of an
AVIOContext's read_packet-function: If it did not read anything,
it returned zero, which currently leads to a warning in read_packet_wrapper
in aviobuf.c. Said warning will be an av_assert2 as soon as
FF_API_OLD_AVIO_EOF_0 is zero (probably the next major version bump).
So instead forward the error code from the underlying protocol.

This error/assert is triggered in the wtv-demux FATE test.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>