control/ts2021: fix data race during concurrent Close and conn ending

Fixes tailscale/corp#33125

Change-Id: I9911f5059d5ebe42ecf7db9becb2326cca240765
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/ts2021/client.go

@@ -180,6 +180,7 @@ func (nc *Client) Close() error {
 	nc.mu.Lock()
 	live := nc.connPool
 	nc.closed = true
+	nc.connPool = nil // stop noteConnClosed from mutating it as we loop over it (in live) below
 	nc.mu.Unlock()
 
 	for _, c := range live {