pgsql: Remove Assert(BgWriterShmem != NULL), which is rather pointless

Log Message:
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Remove Assert(BgWriterShmem != NULL), which is rather pointless since
we'd dump core anyway immediately afterward if it were null; and it
seems to confuse some versions of icc into generating bad code.
Per report from Sergey Koposov. Patched in HEAD only, for the moment,
since this is only likely to affect developers.

Modified Files:
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pgsql/src/backend/postmaster:
bgwriter.c (r1.42 -> r1.43)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c?r1=1.42&r2=1.43)