V4L/DVB (8546): saa7146: fix read from uninitialized memory

The offset field of the scatterlist entry *after* the last valid scatterlist
entry was used instead of the first scatterlist entry (as was the intention
of this code).

This worked fine until the kzalloc of the sglist was replaced with kmalloc
and sg_init_table only zeroed the exact needed length. Apparently kzalloc
zeroes a bit more than is strictly necessary so the offset field was
always 0 in the past.

But now the offset field was suddenly random and this led to broken captures.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>