video: common: prevent divide-by-zero video_closest_frmival_stepwise()

Adds an assertion and runtime log to ensure 'step' is not zero before
using it in division,preventing undefined behavior without modifying
 the public API.

Avoids a signature change (void → int) to preserve API stability for
Zephyr 4.2.A more complete solution can be proposed for 4.3.

CID: 444378

Signed-off-by: sudarsan N <sudarsansamy2002@gmail.com>