svq3: Avoid a division by zero

If the height is zero, the decompression will probably end up
failing due to not fitting into the allocated buffer later
anyway, so this doesn't need any more elaborate check.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
