V4L/DVB (11958): usbvision-core.c: vfree does its own NULL check

vfree() does it's own NULL checking,so no need for check before
calling it.

Signed-off-by: Figo.zhang <figo1802@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>