pgsql: Don't try to fetch database name when SetTransactionIdLimit() is

Don't try to fetch database name when SetTransactionIdLimit() is executed
outside a transaction.

This repairs brain fade in my patch of 2009-08-30: the reason we had been
storing oldest-database name, not OID, in ShmemVariableCache was of course
to avoid having to do a catalog lookup at times when it might be unsafe.

This error explains why Aleksandr Dushein is having trouble getting out of
an XID wraparound state in bug #5718, though not how he got into that state
in the first place.  I suspect pg_upgrade is at fault there.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=c98cd9bdb618794643b670c8240c0aad213340c7

Modified Files
--------------
src/backend/access/transam/varsup.c |   18 ++++++++++++++----
1 files changed, 14 insertions(+), 4 deletions(-)