spark: fix race condition in cDemux

cDemux destructor was racing with Read() which did lead to all
sorts of nasty crashes because after poll returned, the dmx object
could be gone and its memory replaced with totally different things.

libspark/dmx_lib.h

@@ -42,6 +42,7 @@ class cDemux
 		struct dmx_pes_filter_params p_flt;
 		int last_source;
 		bool _open(void);
+		void *pdata;
 	public:
 
 		bool Open(DMX_CHANNEL_TYPE pes_type, void * unused = NULL, int bufsize = 0);