ipn/ipnlocal: fix a panic in setPrefsLockedOnEntry when cc is nil

The AlwaysOn policy can be applied by (*LocalBackend).applySysPolicy, flipping WantRunning from false to true
before (*LocalBackend).Start() has been called for the first time and set a control client in b.cc. This results in a nil
pointer dereference and a panic when setPrefsLockedOnEntry applies the change and calls controlclient.Client.Login().

In this PR, we fix it by only doing a login if b.cc has been set.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/local.go

@@ -4181,7 +4181,7 @@ func (b *LocalBackend) setPrefsLockedOnEntry(newp *ipn.Prefs, unlock unlockOnce)
 		b.MagicConn().SetDERPMap(netMap.DERPMap)
 	}
 
-	if !oldp.WantRunning() && newp.WantRunning {
+	if !oldp.WantRunning() && newp.WantRunning && cc != nil {
 		b.logf("transitioning to running; doing Login...")
 		cc.Login(controlclient.LoginDefault)
 	}