detect/asn1: Fix relative_offset keyword option

detect/asn1: Fix relative_offset keyword option

- Fix relative_offset keyword option to be relative in regards to the
last content match
- Change relative_offset to int32_t with bounds check to allow the full
range of the packet buffer size (uint16_t)
- Added checks for over/underflows
- Changed the offset type to uint16_t because the offset is applied to
the payload length, which is a uint16_t
- Adjusted test cases to work relative to the content match
- Added test case to verify bounds