pgsql: Add simple sanity checks on newly-read pages to GiST, too.

Log Message:
-----------
Add simple sanity checks on newly-read pages to GiST, too.

Modified Files:
--------------
    pgsql/src/backend/access/gist:
        gist.c (r1.127 -> r1.128)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gist.c.diff?r1=1.127&r2=1.128)
        gistget.c (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistget.c.diff?r1=1.52&r2=1.53)
        gistutil.c (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistutil.c.diff?r1=1.7&r2=1.8)
        gistvacuum.c (r1.9 -> r1.10)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gist/gistvacuum.c.diff?r1=1.9&r2=1.10)
    pgsql/src/include/access:
        gist_private.h (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/gist_private.h.diff?r1=1.8&r2=1.9)