Fix race condition when sharing tuple descriptors.

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

src/backend/utils/cache/typcache.c

@@ -2822,7 +2822,7 @@ find_or_make_matching_shared_tupledesc(TupleDesc tupdesc)
 		Assert(record_table_entry->key.shared);
 		result = (TupleDesc)
 			dsa_get_address(CurrentSession->area,
-							record_table_entry->key.shared);
+							record_table_entry->key.u.shared_tupdesc);
 		Assert(result->tdrefcount == -1);
 
 		return result;