kernel: Fix missing reference to team/thread in signal events.

The signal to the team/thread is only actually sent in a deferred
procedure. To ensure that the team/thread stays valid between the DPC
being queued and it actually running, we need to acquire a reference.

Fixes #11390, where the DPC was run after the team was already
destroyed.

headers/private/kernel/UserEvent.h

@@ -52,6 +52,8 @@ struct TeamSignalEvent : SignalEvent {
 	static	TeamSignalEvent*	Create(Team* team, uint32 signalNumber,
 									int32 signalCode, int32 errorCode);
 
+	virtual	status_t			Fire();
+
 protected:
 	virtual	void				DoDPC(DPCQueue* queue);
 
@@ -69,6 +71,8 @@ struct ThreadSignalEvent : SignalEvent {
 									int32 signalCode, int32 errorCode,
 									pid_t sendingTeam);
 
+	virtual	status_t			Fire();
+
 protected:
 	virtual	void				DoDPC(DPCQueue* queue);