pgsql: Fix crash when reading 'timezone = unknown' from postgresql.conf

Log Message:
-----------
Fix crash when reading 'timezone = unknown' from postgresql.conf during
SIGHUP; it's not OK for an assign_hook to return a non-malloc'd string.
Problem was introduced during timezone library rewrite.

Modified Files:
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pgsql/src/backend/commands:
variable.c (r1.112 -> r1.113)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/variable.c.diff?r1=1.112&r2=1.113)