V4L/DVB(12993b): gl860: Prevent a potential risk of zeroing a floating pointer

drivers/media/video/gspca/gl860/gl860.c: In function ‘gl860_build_control_table’:
drivers/media/video/gspca/gl860/gl860.c:119: warning: ‘sd_ctrls’ may be used uninitialized in this function

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