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.GA22278@alap3.anarazel.de
Backpatch: 9.5, where the bug was introduced as part of 4f627f

src/backend/access/transam/multixact.c

@@ -2552,6 +2552,7 @@ SetOffsetVacuumLimit(void)
 	bool		oldestOffsetKnown = false;
 	bool		prevOldestOffsetKnown;
 	MultiXactOffset offsetStopLimit = 0;
+	MultiXactOffset prevOffsetStopLimit;
 
 	/*
 	 * NB: Have to prevent concurrent truncation, we might otherwise try to
@@ -2566,6 +2567,7 @@ SetOffsetVacuumLimit(void)
 	nextOffset = MultiXactState->nextOffset;
 	prevOldestOffsetKnown = MultiXactState->oldestOffsetKnown;
 	prevOldestOffset = MultiXactState->oldestOffset;
+	prevOffsetStopLimit = MultiXactState->offsetStopLimit;
 	Assert(MultiXactState->finishedStartup);
 	LWLockRelease(MultiXactGenLock);
 
@@ -2633,11 +2635,13 @@ SetOffsetVacuumLimit(void)
 	{
 		/*
 		 * If we failed to get the oldest offset this time, but we have a
-		 * value from a previous pass through this function, use the old value
-		 * rather than automatically forcing it.
+		 * value from a previous pass through this function, use the old
+		 * values rather than automatically forcing an emergency autovacuum
+		 * cycle again.
 		 */
 		oldestOffset = prevOldestOffset;
 		oldestOffsetKnown = true;
+		offsetStopLimit = prevOffsetStopLimit;
 	}
 
 	/* Install the computed values */