bspatch: add integer overflow checks

Introduce a new add_off_t static function that exits with an error
message if there's an overflow, otherwise returns their sum. Use this
when adding values obtained from the input patch.

Reviewed by: delphij, allanjude (earlier)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7897