media: saa7134: improve f->fmt.win.clips NULL check

Fix this smatch error:

saa7134-video.c:1286 saa7134_g_fmt_vid_overlay() error: we previously assumed 'f->fmt.win.clips' could be null (see line 1279)

This is actually a false error since if f->fmt.win.clips is NULL, clipcount
will be set to 0, so the clips array won't be touched, but it doesn't hurt
to make this explicit in the code.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>