fix for _copyUnique()

I have commited a fix for _copyUnique(nodes/copyfuncs.c) suggested by
Hiroshi Inoue. It seems obvious for me and passed regression tests.
--
Tatsuo Ishii

*** backend/nodes/copyfuncs.c.orig	Mon Apr 19 16:00:40 1999
--- backend/nodes/copyfuncs.c	Mon Apr 26 13:34:26 1999
***************
*** 514,520 ****
  	 *	copy remainder of node
  	 * ----------------
  	 */
! 	if (newnode->uniqueAttr)
  		newnode->uniqueAttr = pstrdup(from->uniqueAttr);
  	else
  		newnode->uniqueAttr = NULL;
--- 514,520 ----
  	 *	copy remainder of node
  	 * ----------------
  	 */
! 	if (from->uniqueAttr)
  		newnode->uniqueAttr = pstrdup(from->uniqueAttr);
  	else
  		newnode->uniqueAttr = NULL;