pgsql-server: Must guard against NULL return from localtime() when

Log Message:
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Must guard against NULL return from localtime() when probing pre-1970
dates. Per Magnus Hagander.

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