avplay: remove a warning

Set the data field in the flush_pkt to the pointer to the actual packet.
The field needs to contain a valid unique pointer, no read nor writes
are ever made to it.

avplay.c

@@ -2963,7 +2963,7 @@ int main(int argc, char **argv)
     SDL_EventState(SDL_USEREVENT, SDL_IGNORE);
 
     av_init_packet(&flush_pkt);
-    flush_pkt.data = "FLUSH";
+    flush_pkt.data = (uint8_t *)&flush_pkt;
 
     cur_stream = stream_open(input_filename, file_iformat);