Fix non-atomic set of logging.filterLength.

One of its accesses was not atomic; tsan caught it.
diff --git a/glog.go b/glog.go
index 55e31e9..da3c6a7 100644
--- a/glog.go
+++ b/glog.go
@@ -470,7 +470,7 @@
 	// Turn verbosity off so V will not fire while we are in transition.
 	logging.verbosity.set(0)
 	// Ditto for filter length.
-	logging.filterLength = 0
+	atomic.StoreInt32(&logging.filterLength, 0)
 
 	// Set the new filters and wipe the pc->Level map if the filter has changed.
 	if setFilter {