pgsql: Prevent show_session_authorization from crashing when session_au

Prevent show_session_authorization from crashing when session_authorization
hasn't been set.

The only known case where this can happen is when show_session_authorization
is invoked in an autovacuum process, which is possible if an index function
calls it, as for example in bug #5669 from Andrew Geery. We could perhaps
try to return a sensible value, such as the name of the cluster-owning
superuser; but that seems like much more trouble than the case is worth,
and in any case it could create new possible failure modes. Simply
returning an empty string seems like the most appropriate fix.

Back-patch to all supported versions, even those before autovacuum, just
in case there's another way to provoke this crash.

Branch
------
REL8_1_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=511449d40fd48ba97556a03c272fd66b9c2c3f9c

Modified Files
--------------
src/backend/commands/variable.c | 4 ++++
1 files changed, 4 insertions(+), 0 deletions(-)