Fix another instance of unsafe coding for shm_toc_lookup failure.

Fix another instance of unsafe coding for shm_toc_lookup failure.

One or another author of commit 5bcf389ec seems to have thought that
computing an offset from a NULL pointer would yield another NULL pointer.
There may possibly be architectures where that works, but common machines
don't work like that.  Per a quick code review of places calling
shm_toc_lookup and not using noError = false.