state_checker: when checking GC last data, don't create new ops

Because this happens during shut down, the new ops will never be
cleaned up!  We only care about ones that actually ran GC here, so no
need to create new ones.

libkbfs/state_checker.go

@@ -113,8 +113,11 @@ func (sc *StateChecker) getLastGCData(ctx context.Context,
 	var latestTime time.Time
 	var latestRev kbfsmd.Revision
 	for _, c := range *config.allKnownConfigsForTesting {
-		ops := c.KBFSOps().(*KBFSOpsStandard).getOps(context.Background(),
-			FolderBranch{tlfID, MasterBranch}, FavoritesOpNoChange)
+		ops := c.KBFSOps().(*KBFSOpsStandard).getOpsIfExists(
+			context.Background(), FolderBranch{tlfID, MasterBranch})
+		if ops == nil {
+			continue
+		}
 		rt, rev := ops.fbm.getLastQRData()
 		if rt.After(latestTime) && rev > latestRev {
 			latestTime = rt