Calling dotrap() can do almost anything, including reallocating the
jobtab array. Convert the job pointer to an index before calling
dotrap() and then restore a proper job pointer afterwards.
PR: 290330
Reported by: bdrewery
Reviewed by: bdrewery
Differential Revision: https://reviews.freebsd.org/D53793
(cherry picked from commit f44ac8cc9c)