If no DSM slots are available, a ParallelContext can still be
created, but its seg pointer is NULL.  Teach parallel btree build
to cope with that by falling back to a regular non-parallel build,
to avoid crashing with a segmentation fault.

Back-patch to 11, where parallel CREATE INDEX landed.

Reported-by: Nicola Contu
Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CA%2BhUKGJgJEBnkuODBVomyK3MWFvDBbMVj%3Dgdt6DnRPU-5sQ6UQ%40mail.gmail.com