derp/derphttp: fix nil pointer dereference when closing a netcheck client

NewNetcheckClient only initializes a subset of fields of derphttp.Client,
and the Close() call added by #5707 was result in a nil pointer dereference.
Make Close() safe to call when using NewNetcheckClient() too.

Fixes #5919

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

derp/derphttp/derphttp_client.go

@@ -96,7 +96,7 @@ func NewRegionClient(privateKey key.NodePrivate, logf logger.Logf, getRegion fun
 	return c
 }
 
-// NewNetcheckClient returns a Client that's only able to have its DialRegion method called.
+// NewNetcheckClient returns a Client that's only able to have its DialRegionTLS method called.
 // It's used by the netcheck package.
 func NewNetcheckClient(logf logger.Logf) *Client {
 	return &Client{logf: logf}
@@ -985,7 +985,9 @@ func (c *Client) isClosed() bool {
 // Close closes the client. It will not automatically reconnect after
 // being closed.
 func (c *Client) Close() error {
+	if c.cancelCtx != nil {
 		c.cancelCtx() // not in lock, so it can cancel Connect, which holds mu
+	}
 
 	c.mu.Lock()
 	defer c.mu.Unlock()