ipn/ipnlocal: fix nil control client panic while updating TKA head

As part of tailnet-lock netmap processing, the LocalBackend mutex
is unlocked so we can potentially make a network call. Its possible
(during shutdown or while the control client is being reset) for
b.cc to become nil before the lock is picked up again.

Fixes: #9554
Signed-off-by: Tom DNetto <tom@tailscale.com>

ipn/ipnlocal/local.go

@@ -1120,15 +1120,19 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 			b.logf("[v1] TKA sync error: %v", err)
 		}
 		b.mu.Lock()
-		if b.tka != nil {
-			head, err := b.tka.authority.Head().MarshalText()
-			if err != nil {
-				b.logf("[v1] error marshalling tka head: %v", err)
+		// As we stepped outside of the lock, it's possible for b.cc
+		// to now be nil.
+		if b.cc != nil {
+			if b.tka != nil {
+				head, err := b.tka.authority.Head().MarshalText()
+				if err != nil {
+					b.logf("[v1] error marshalling tka head: %v", err)
+				} else {
+					b.cc.SetTKAHead(string(head))
+				}
 			} else {
-				b.cc.SetTKAHead(string(head))
+				b.cc.SetTKAHead("")
 			}
-		} else {
-			b.cc.SetTKAHead("")
 		}
 
 		if !envknob.TKASkipSignatureCheck() {