pgsql: Code cleanup.

Log Message:
-----------
Code cleanup. gistfillbuffer accepts InvalidOffsetNumber.

Modified Files:
--------------
pgsql/src/backend/access/gist:
gist.c (r1.122 -> r1.123)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gist.c.diff?r1=1.122&r2=1.123)
gistutil.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistutil.c.diff?r1=1.3&r2=1.4)
gistvacuum.c (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistvacuum.c.diff?r1=1.3&r2=1.4)
gistxlog.c (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistxlog.c.diff?r1=1.4&r2=1.5)