jffsX-utils: fix integer overflow in jffs2dump.c

jffsX-utils: fix integer overflow in jffs2dump.c

Report of the static analyzer:
The value of an arithmetic expression 'datsize + oobsize' is a subject to overflow because its operands are not cast to a larger data type before performing arithmetic.

Corrections explained:
- Added a check to validate datsize and oobsize to ensure they are non-negative and within a safe range.
- Cast datsize and oobsize to long before performing arithmetic to prevent potential integer overflow.

This change ensures safe computation of offsets and prevents undefined behavior caused by overflow.

Signed-off-by: Anton Moryakov <ant.v.moryakov@gmail.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>