[media] drivers/media/pci/saa7134/saa7134-dvb.c: Test if videobuf_dvb_get_frontend return NULL

Based on commit: e66131ce
Not testing videobuf_dvb_get_frontend output may cause OOPS if it return
NULL. This patch fixes this issue.
The semantic patch that found this issue is(http://coccinelle.lip6.fr/

):
// <smpl>
@@
identifier i,a,b;
statement S, S2;
@@
i = videobuf_dvb_get_frontend(...);
... when != if (!i) S
* if (i->a.b)
S2
// </smpl>

Signed-off-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>