[media] s5p-fimc: Avoid possible NULL pointer dereference in set_fmt op

This fixes following issue found with a static analysis tool:
Pointer 'ffmt' returned from call to function 'fimc_capture_try_format'
at line 1522 may be NULL and may be dereferenced at line 1535.
Although it shouldn't happen in practice, add the NULL pointer check
to be on the safe side.

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>