Avoid dereferencing of NULL f->fmt. Correct size of the allocated
buffer in case the crop rectangle is smaller than the bounds
rectangle (configured with S_FMT). Also remove redundant check
for *num_buffer == 0 as this case is handled in videobuf2.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>