pgsql: Don't try to close negative file descriptors, since this can

Log Message:
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Don't try to close negative file descriptors, since this can cause
crashes on certain platforms. In particular, the MSVC runtime is known
to do this.

Fixes bug #4162, reported and diagnosed by Javier Pimas

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/src/backend/access/transam:
xlog.c (r1.292.2.3 -> r1.292.2.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.292.2.3&r2=1.292.2.4)