pgsql: Fix PageGetExactFreeSpace() so that it actually behaves sensibly

Log Message:
-----------
Fix PageGetExactFreeSpace() so that it actually behaves sensibly
if pd_lower > pd_upper, rather than merely claiming to.  This would
only matter if the page header were corrupt, which shouldn't occur,
but ...

Modified Files:
--------------
    pgsql/src/backend/storage/page:
        bufpage.c (r1.77 -> r1.78)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/page/bufpage.c?r1=1.77&r2=1.78)