derp/derphttp: use a getter method to read server key

To hold the mutex while accessing it.

Fixes #10122

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

derp/derphttp/derphttp_client.go

@@ -100,7 +100,7 @@ type Client struct {
 }
 
 func (c *Client) String() string {
-	return fmt.Sprintf("<derphttp_client.Client %s url=%s>", c.serverPubKey.ShortString(), c.url)
+	return fmt.Sprintf("<derphttp_client.Client %s url=%s>", c.ServerPublicKey().ShortString(), c.url)
 }
 
 // NewRegionClient returns a new DERP-over-HTTP client. It connects lazily.