ssh: fix spinloop in mux SendRequest drain on closed channel

The drain loop added in 4e7a738 ("ssh: fix deadlock on unexpected
global responses") receives from m.globalResponses without checking
the comma-ok flag. Once mux.loop closes globalResponses on connection
teardown, the loop spins forever on the zero value, pinning a CPU
core for every concurrent SendRequest caller.

Use the comma-ok idiom to detect the closed channel and break out.

Change-Id: Icf0a7419fe5b206b7a4f70429fa9f25d0b5af731
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/783681
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Neal Patel <neal@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

ssh/mux.go

@@ -155,7 +155,10 @@ func (m *mux) SendRequest(name string, wantReply bool, payload []byte) (bool, []
 	drain:
 		for {
 			select {
-			case <-m.globalResponses:
+			case _, ok := <-m.globalResponses:
+				if !ok {
+					break drain
+				}
 			default:
 				break drain
 			}