pgsql: Change Assert() to a plain test and elog, just to see if that

Log Message:
-----------
Change Assert() to a plain test and elog, just to see if that works
around the icc bug exhibited by buildfarm member dugong.

Modified Files:
--------------
pgsql/src/backend/postmaster:
bgwriter.c (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c?r1=1.44&r2=1.45)