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;