win_state: silence a valgrind warning

m_geometry_apply() will read and modify the dummy variable. It's not
actually used for anything, but valgrind will still warn against
uninitialized data. I'm not sure whether this was UB, but in any case
it's annoying when running valgrind.

video/out/win_state.c

@@ -42,7 +42,7 @@ static void apply_autofit(int *w, int *h, int scr_w, int scr_h,
     if (!geo->wh_valid)
         return;
 
-    int dummy;
+    int dummy = 0;
     int n_w = *w, n_h = *h;
     m_geometry_apply(&dummy, &dummy, &n_w, &n_h, scr_w, scr_h, geo);