net/rpc: unlock client.mutex as early as possible

Although these changes have no essential influence, I think this is a better point.

Change-Id: I571d3a14c948d2fd7bc9561f47f33e9e4c90683f
GitHub-Last-Rev: d8c5d180069bb91b6eb1247769bd14f728922fc0
GitHub-Pull-Request: golang/go#24697
Reviewed-on: https://go-review.googlesource.com/104895Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/rpc/client.go

@@ -75,8 +75,8 @@ func (client *Client) send(call *Call) {
 	// Register this call.
 	client.mutex.Lock()
 	if client.shutdown || client.closing {
-		call.Error = ErrShutdown
 		client.mutex.Unlock()
+		call.Error = ErrShutdown
 		call.done()
 		return
 	}