storage/flash_map: Fix is_in_flash_area_bounds

Prevent possible overflow in is_in_flash_area_bounds while
validating offset and length of an operation.

Fixes #89349

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
(cherry picked from commit 3d4b4272457a353e386b048f8c6ff7ab7b38c2e3)
1 file changed