Avoid core dump in _outPathInfo() for Path without a parent RelOptInfo.

Nearly all Paths have parents, but a ResultPath representing an empty FROM
clause does not.  Avoid a core dump in such cases.  I believe this is only
a hazard for debugging usage, not for production, else we'd have heard
about it before.  Nonetheless, back-patch to 9.1 where the troublesome code
was introduced.  Noted while poking at bug #11703.

src/backend/nodes/outfuncs.c

@@ -1466,7 +1466,10 @@ _outPathInfo(StringInfo str, Path *node)
 {
 	WRITE_ENUM_FIELD(pathtype, NodeTag);
 	appendStringInfo(str, " :parent_relids ");
-	_outBitmapset(str, node->parent->relids);
+	if (node->parent)
+		_outBitmapset(str, node->parent->relids);
+	else
+		_outBitmapset(str, NULL);
 	WRITE_FLOAT_FIELD(startup_cost, "%.2f");
 	WRITE_FLOAT_FIELD(total_cost, "%.2f");
 	WRITE_NODE_FIELD(pathkeys);