pgsql-server: Adjust PageGetMaxOffsetNumber to ensure sane behavior on

Log Message:
-----------
Adjust PageGetMaxOffsetNumber to ensure sane behavior on uninitialized
pages, even when the macro's result is stored into an unsigned variable.

Modified Files:
--------------
pgsql-server/src/include/storage:
bufpage.h (r1.57 -> r1.58)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/bufpage.h.diff?r1=1.57&r2=1.58)