V4L/DVB (11225): v4lgrab: fix compilation warnings

Documentation/video4linux/v4lgrab.c: In function ‘main’:
Documentation/video4linux/v4lgrab.c:193: warning: ‘src_depth’ is used uninitialized in this function
Documentation/video4linux/v4lgrab.c:108: warning: ‘b’ may be used uninitialized in this function
Documentation/video4linux/v4lgrab.c:108: warning: ‘g’ may be used uninitialized in this function
Documentation/video4linux/v4lgrab.c:108: warning: ‘r’ may be used uninitialized in this function

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>