avformat/rm: limit packet size

The chunk size is limited to 0xFFFF (written by avio_wb16), so make
sure that the packet size is not too large.

Such large frames need to be split into slices smaller than 64 kB, but
that is currently supported neither by the rv10/rv20 encoders nor the rm
muxer.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

See Ticket244

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
