Protect against NULL-dereference in pg_dump

findTableByOid() is allowed to return NULL and we should therefore be
checking for that case.  getOwnedSeqs() and dumpSequence() shouldn't
ever actually see this happen, but given odd circumstances it might and
commit f9e439b1 probably shouldn't have removed that check.

Pointed out by Coverity.  Initial patch from Michael Paquier.

Back-patch to 9.6, where that commit had removed the check.

src/bin/pg_dump/pg_dump.c

@@ -5643,6 +5643,9 @@ getOwnedSeqs(Archive *fout, TableInfo tblinfo[], int numTables)
 			continue;			/* not an owned sequence */
 
 		owning_tab = findTableByOid(seqinfo->owning_tab);
+		if (owning_tab == NULL)
+			exit_horribly(NULL, "failed sanity check, parent table OID %u of sequence OID %u not found\n",
+						  seqinfo->owning_tab, seqinfo->dobj.catId.oid);
 
 		/*
 		 * We need to dump the components that are being dumped for the table
@@ -15583,7 +15586,11 @@ dumpSequence(Archive *fout, TableInfo *tbinfo)
 	{
 		TableInfo  *owning_tab = findTableByOid(tbinfo->owning_tab);
 
-		if (owning_tab && owning_tab->dobj.dump & DUMP_COMPONENT_DEFINITION)
+		if (owning_tab == NULL)
+			exit_horribly(NULL, "failed sanity check, parent table OID %u of sequence OID %u not found\n",
+						  tbinfo->owning_tab, tbinfo->dobj.catId.oid);
+
+		if (owning_tab->dobj.dump & DUMP_COMPONENT_DEFINITION)
 		{
 			resetPQExpBuffer(query);
 			appendPQExpBuffer(query, "ALTER SEQUENCE %s",