pgsql: Tweak the tree descent loop in fsm_search_avail to not look at

Log Message:
-----------
Tweak the tree descent loop in fsm_search_avail to not look at the
right child if it doesn't need to.  This saves some miniscule number
of cycles, but the ulterior motive is to avoid an optimization bug
known to exist in SCO's C compiler (and perhaps others?)

Modified Files:
--------------
    pgsql/src/backend/storage/freespace:
        fsmpage.c (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/freespace/fsmpage.c?r1=1.2&r2=1.3)