xwddec: prevent overflow of lsize * avctx->height

This is used to check if the input buffer is larger enough, so if this
overflows it can cause a false negative leading to a segmentation fault
in bytestream2_get_bufferu.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>