ffplay: fix order of setting show_mode

Without the fix the refresh event may have got called with unset show mode.

Fixes ticket #2174.
Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -2641,10 +2641,11 @@ static int read_thread(void *arg)
     if (st_index[AVMEDIA_TYPE_VIDEO] >= 0) {
         ret = stream_component_open(is, st_index[AVMEDIA_TYPE_VIDEO]);
     }
-    is->refresh_tid = SDL_CreateThread(refresh_thread, is);
     if (is->show_mode == SHOW_MODE_NONE)
         is->show_mode = ret >= 0 ? SHOW_MODE_VIDEO : SHOW_MODE_RDFT;
 
+    is->refresh_tid = SDL_CreateThread(refresh_thread, is);
+
     if (st_index[AVMEDIA_TYPE_SUBTITLE] >= 0) {
         stream_component_open(is, st_index[AVMEDIA_TYPE_SUBTITLE]);
     }