sh: Fix job pointer invalidation with trapsasync

[jilles]

Summary

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

Test Plan

Install patched sh with new tests, cd /usr/tests/bin/sh && kyua test and reboot (all within a virtual machine).

[bdrewery]

Thank you