wgengine/magicsock: ensure that an existing timer is stopped

There presently is not a bug here, but this is a time-bomb waiting for
an innocent incremental change to accidentally adjust the parent call
chain resulting in a timer leak.

Updates #cleanup
Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -4441,7 +4441,10 @@ func (de *endpoint) heartbeat() {
 	de.mu.Lock()
 	defer de.mu.Unlock()
 
-	de.heartBeatTimer = nil
+	if de.heartBeatTimer != nil {
+		de.heartBeatTimer.Stop()
+		de.heartBeatTimer = nil
+	}
 
 	if de.heartbeatDisabled {
 		// If control override to disable heartBeatTimer set, return early.