pgsql-server: Avoid crashing when restoring a saved GUC

Log Message:
-----------
Avoid crashing when restoring a saved GUC session_authorization value
that refers to a now-deleted userid. Per gripe from Chris Ochs.

Tags:
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REL7_4_STABLE

Modified Files:
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pgsql-server/src/backend/commands:
variable.c (r1.88 -> r1.88.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/variable.c.diff?r1=1.88&r2=1.88.2.1)
pgsql-server/src/backend/utils/misc:
guc.c (r1.164.2.2 -> r1.164.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/utils/misc/guc.c.diff?r1=1.164.2.2&r2=1.164.2.3)