wgengine/magicsock: do not apply node view updates to a closed Conn (#17517)

Fixes #17516

Change-Id: Iae2dab42d6f7bc618478d360a1005537c1fa1bbd
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -2958,8 +2958,13 @@ func (c *Conn) onNodeViewsUpdate(update NodeViewsUpdate) {
 	filt := c.filt
 	self := c.self
 	peers := c.peers
+	isClosed := c.closed
 	c.mu.Unlock() // release c.mu before potentially calling c.updateRelayServersSet which is O(m * n)
 
+	if isClosed {
+		return // nothing to do here, the conn is closed and the update is no longer relevant
+	}
+
 	if peersChanged || relayClientChanged {
 		if !relayClientEnabled {
 			c.relayManager.handleRelayServersSet(nil)