Parallel query processes that called BlessTupleDesc() for identical
tuple descriptors at the same moment could crash.  There was code to
handle that rare case, but it dereferenced a bogus DSA pointer.  Repair.

Back-patch to 11, where commit cc5f8136 added support for sharing tuple
descriptors in parallel queries.
Reported-by: Eric Thinnes <e.thinnes@gmx.de>
Discussion: https://postgr.es/m/99aaa2eb-e194-bf07-c29a-1a76b4f2bcf9%40gmx.de