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

@@ -1120,6 +1120,9 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 			b.logf("[v1] TKA sync error: %v", err)
 		}
 		b.mu.Lock()
+		// 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 {
@@ -1130,6 +1133,7 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 			} else {
 				b.cc.SetTKAHead("")
 			}
+		}
 
 		if !envknob.TKASkipSignatureCheck() {
 			b.tkaFilterNetmapLocked(st.NetMap)