control/controlclient: handle errors in rememberLastNetmapUpdator (#19112)

If errors occured, the updater could end up deadlocked.

Closing the done channel rather than adding to it, fixes a deadlock in
the corp tests.

Updates #19111

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

control/controlclient/map_test.go

@@ -669,7 +669,7 @@ func TestUpdateDiscoForNode(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			nu := &rememberLastNetmapUpdater{
-				done: make(chan any),
+				done: make(chan any, 1),
 			}
 			ms := newTestMapSession(t, nu)