V4L/DVB (3420): Nskips maybe used uninitialized in bttv_risc_overlay

The Coverity checker (previously Stanford checker) noticed that
the value of nskips could be read even if it was never written.

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>