[VOL-4442] grpc streaming connection monitoring

Change-Id: Id787e94cf28745d36e72f8ed2f5c316312714db4
diff --git a/vendor/google.golang.org/grpc/pickfirst.go b/vendor/google.golang.org/grpc/pickfirst.go
index f194d14..5168b62 100644
--- a/vendor/google.golang.org/grpc/pickfirst.go
+++ b/vendor/google.golang.org/grpc/pickfirst.go
@@ -125,7 +125,7 @@
 }
 
 func (b *pickfirstBalancer) ExitIdle() {
-	if b.state == connectivity.Idle {
+	if b.sc != nil && b.state == connectivity.Idle {
 		b.sc.Connect()
 	}
 }