lavc/libxvid: workaround for bug in libxvidcore

libxvidcore calculate number of threads basing on video height.
If height is small enough it allocates 0 bytes long memory and
writes to it.
Setting thread_count to 0 uses 1 thread and skips bugged code.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>