refactor: #define DEAD_HEIGHT -8

From the research conducted in
https://github.com/ioquake/ioq3/issues/794
we can probably conclude that the `self->r.maxs[2] = -8;`
in `player_die()` is related to `pm->maxs[2] = -8;`
in `PM_CheckDuck()`. Let's make this clear with a named constant.

code/game/bg_pmove.c

@@ -1275,7 +1275,7 @@ static void PM_CheckDuck (void)
 
 	if (pm->ps->pm_type == PM_DEAD)
 	{
-		pm->maxs[2] = -8;
+		pm->maxs[2] = DEAD_HEIGHT;
 		pm->ps->viewheight = DEAD_VIEWHEIGHT;
 		return;
 	}