human_player_gui.c: fix segfault when selecting one unit

Checking against `sel` is just plain wrong, since it always expected to
be non-NULL, since `h->sel` is an array defined in `struct
human_player`. Instead, `sel->d.i` is the pointer that should be
checked.

src/player/src/human_player_gui.c

@@ -206,7 +206,7 @@ static int render_sel_single(const struct human_player *const h,
     {
         const struct sel_instance *const sel = &h->sel[i];
 
-        if (sel)
+        if (sel->d.i)
         {
             switch (sel->type)
             {