replay: fix potential invalid memory access

_subscriptions is a vector that is resized when needed. However there could
still be references to elements in the vector when the resize happens.
These references then become invalid.
Using a vector of pointers fixes that.

src/modules/replay/replay.hpp

@@ -58,9 +58,9 @@ namespace px4
 class Replay : public ModuleBase<Replay>
 {
 public:
-	Replay() {}
+	Replay() = default;
 
-	virtual ~Replay() {}
+	virtual ~Replay();
 
 	/** @see ModuleBase */
 	static int task_spawn(int argc, char *argv[]);
@@ -205,7 +205,7 @@ protected:
 	 */
 	bool nextDataMessage(std::ifstream &file, Subscription &subscription, int msg_id);
 
-	std::vector<Subscription> _subscriptions;
+	std::vector<Subscription *> _subscriptions;
 	std::vector<uint8_t> _read_buffer;
 
 private: