pgsql: Fix GiST's killing tuple: GISTScanOpaque->curpos wasn't

Log Message:
-----------
Fix GiST's killing tuple: GISTScanOpaque->curpos wasn't
correctly set. As result, killtuple() marks as dead
wrong tuple on page. Bug was introduced by me while fixing
possible duplicates during GiST index scan.

Tags:
----
REL8_3_STABLE

Modified Files:
--------------
    pgsql/src/backend/access/gist:
        gistget.c (r1.69.2.2 -> r1.69.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistget.c?r1=1.69.2.2&r2=1.69.2.3)
        gistscan.c (r1.68.2.2 -> r1.68.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistscan.c?r1=1.68.2.2&r2=1.68.2.3)
    pgsql/src/include/access:
        gist_private.h (r1.28.2.2 -> r1.28.2.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h?r1=1.28.2.2&r2=1.28.2.3)