pgsql: Ensure that any memory leaked during an error inside the bgwriter

Log Message:
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Ensure that any memory leaked during an error inside the bgwriter is
recovered.  I did not see any actual leak while testing this in CVS tip,
but 8.0 definitely has a problem with leaking the space temporarily
palloc'd by BufferSync().  In any case this seems a good idea to forestall
similar problems in future.  Per report from Arjen van der Meijden.

Modified Files:
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    pgsql/src/backend/postmaster:
        bgwriter.c (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.19&r2=1.20)