media: atomisp: get rid of a warning

On smatch, this warning is trigged:

	drivers/staging/media/atomisp/pci/atomisp2/hmm/hmm_bo.c:324 __bo_take_off_handling() error: we previously assumed 'bo->prev' could be null (see line 314)

Because it can't properly analize the truth table for the above
function. So, add an explicit check for the final condition there.

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