Clear dangling pointer to avoid bogus EXPLAIN printout in a corner case.

ExecReScanHashJoin will destroy the join's hash table if it expects
that the inner relation will produce different rows on rescan.
Up to now it's not bothered to clear the additional pointer to that
hash table that exists in the child HashState node.  However, it's
possible for the query to terminate without building a fresh hash
table (this happens if the outer relation is found to be empty
during the final rescan).  So we can end with a dangling pointer
to a deleted hash table.  That was harmless originally, but since
9.0 EXPLAIN ANALYZE has used that pointer to print hash table
statistics.  In debug builds this reproducibly results in garbage
statistics.  In non-debug builds there's frequently no ill effects,
but in principle one could get wrong EXPLAIN ANALYZE output, or
perhaps even a crash if free() has released the hashtable memory
back to the OS.

To fix, just make sure we clear the additional pointer when destroying
the hash table.  In problematic cases, EXPLAIN ANALYZE will then print
no hashtable statistics (reverting to its pre-9.0 behavior).  This isn't
ideal, but since the problem manifests only in unusual corner cases,
it's hard to justify taking any risks to do better in the back
branches.  A follow-on patch will improve matters in HEAD.

Konstantin Knizhnik and Tom Lane, per diagnosis by Thomas Munro
of a trouble report from Alvaro Herrera.

Discussion: https://postgr.es/m/20200323165059.GA24950@alvherre.pgsql

src/backend/executor/nodeHashjoin.c

@@ -1336,6 +1336,12 @@ ExecReScanHashJoin(HashJoinState *node)
 		else
 		{
 			/* must destroy and rebuild hash table */
+			HashState  *hashNode = castNode(HashState, innerPlanState(node));
+
+			/* for safety, be sure to clear child plan node's pointer too */
+			Assert(hashNode->hashtable == node->hj_HashTable);
+			hashNode->hashtable = NULL;
+
 			ExecHashTableDestroy(node->hj_HashTable);
 			node->hj_HashTable = NULL;
 			node->hj_JoinState = HJ_BUILD_HASHTABLE;