transport: Test for released messages earlier

This moves the check for released messages earlier in the Release() call
for transport messages. This prevents triggering the race detector, due
to the check happening before an attempt is made at accessing the
message (which involves indirection through a segment that may have been
released already).

While the prior code would not have actually caused a fault in current
code (because the conditional checks both for the released flag and
whether the message is nil), checking by the release flag first is
more correct and may prevent future bugs.

rpc/transport/transport.go

@@ -218,7 +218,10 @@ type outgoingMsg struct {
 }
 
 func (o *outgoingMsg) Release() {
-	if m := o.message.Message(); !o.released && m != nil {
+	if o.released {
+		return
+	}
+	if m := o.message.Message(); m != nil {
 		o.released = true
 		m.Release()
 	}
@@ -246,7 +249,10 @@ func (i *incomingMsg) Message() rpccp.Message {
 }
 
 func (i *incomingMsg) Release() {
-	if m := i.Message().Message(); !i.released && m != nil {
+	if i.released {
+		return
+	}
+	if m := i.Message().Message(); m != nil {
 		i.released = true
 		m.Release()
 	}