pgsql: Fix bug in SetOffsetVacuumLimit() triggered by find_multixact_st

Fix bug in SetOffsetVacuumLimit() triggered by find_multixact_start() failure.

Previously, if find_multixact_start() failed, SetOffsetVacuumLimit() would
install 0 into MultiXactState->offsetStopLimit if it previously succeeded.
Luckily, there are no known cases where find_multixact_start() will return
an error in 9.5 and above. But if it were to happen, for example due to
filesystem permission issues, it'd be somewhat bad: GetNewMultiXactId()
could continue allocating mxids even if close to a wraparound, or it could
erroneously stop allocating mxids, even if no wraparound is looming. The
wrong value would be corrected the next time SetOffsetVacuumLimit() is
called, or by a restart.

Reported-By: Noah Misch, although this is not his preferred fix
Discussion: 20151210140450(dot)GA22278(at)alap3(dot)anarazel(dot)de
Backpatch: 9.5, where the bug was introduced as part of 4f627f

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/ccde00b9b97ed8b7307e39f95bd48922bf955bb2

Modified Files
--------------
src/backend/access/transam/multixact.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)