pgsql: Prevent lazy_space_alloc from making requests that exceed

Log Message:
-----------
Prevent lazy_space_alloc from making requests that exceed MaxAllocSize,
per report from Stefan Kaltenbrunner.

Modified Files:
--------------
pgsql/src/backend/commands:
vacuumlazy.c (r1.66 -> r1.67)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c.diff?r1=1.66&r2=1.67)