V4L/DVB: videobuf-dma-sg: Avoid using a wrong size

drivers/media/video/videobuf-dma-sg.c: In function ‘__videobuf_mmap_mapper’:
drivers/media/video/videobuf-dma-sg.c:557: warning: ‘size’ may be used uninitialized in this function

The condition where size is floating should never happen, due to the paranoia check,
but a future change at the logic might break it. So, let's just set size to zero and
use it for the paranoia check.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>