crypto/tls: make config.Clone return nil if the source is nil

Fixes #40565

Change-Id: I13a67be193f8cd68df02b8729529e627a73d364b
GitHub-Last-Rev: b03d2c04fd88db909b40dfd7bd08fe13d8994ab9
GitHub-Pull-Request: golang/go#40566
Reviewed-on: https://go-review.googlesource.com/c/go/+/246637
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Trust: Emmanuel Odeke <emm.odeke@gmail.com>

common.go

@@ -727,9 +727,12 @@ func (c *Config) ticketKeyFromBytes(b [32]byte) (key ticketKey) {
 // ticket, and the lifetime we set for tickets we send.
 const maxSessionTicketLifetime = 7 * 24 * time.Hour
 
-// Clone returns a shallow clone of c. It is safe to clone a Config that is
+// Clone returns a shallow clone of c or nil if c is nil. It is safe to clone a Config that is
 // being used concurrently by a TLS client or server.
 func (c *Config) Clone() *Config {
+	if c == nil {
+		return nil
+	}
 	c.mutex.RLock()
 	defer c.mutex.RUnlock()
 	return &Config{

tls_test.go

@@ -841,6 +841,13 @@ func TestCloneNonFuncFields(t *testing.T) {
 	}
 }
 
+func TestCloneNilConfig(t *testing.T) {
+	var config *Config
+	if cc := config.Clone(); cc != nil {
+		t.Fatalf("Clone with nil should return nil, got: %+v", cc)
+	}
+}
+
 // changeImplConn is a net.Conn which can change its Write and Close
 // methods.
 type changeImplConn struct {