This protects against a race with an upcall in the parent during the
fork which can clobber the parent's tcb before the vm space is copied
in the child. The child then gets a corrupted tcb that is either null
or that points to another thread that doesn't exist in the child (after
a fork, only the fork()ing thread exists in the child).
Reported by: Arno J. Klaassen (arno at heho / snv / jussieu / fr)
32cdd5f202