Fix d_a_obj_stone bound crashing due to accessing a field improperly

Different vtable layout on mwerks made this direct pointer cast not work. access the field instead.

Also I went searching if there's any other cases of this bug, didn't find any but did find something else to clean up.

src/d/actor/d_a_obj_stone.cpp

@@ -126,7 +126,7 @@ static f32 bound(cXyz* param_0, cBgS_PolyInfo const& param_1, f32 param_2) {
         cXyz pos;
         f32 abs = param_0->abs();
 
-        C_VECReflect(param_0, (Vec*)&plane, &pos);
+        C_VECReflect(param_0, &plane.mNormal, &pos);
         *param_0 = pos * abs * param_2;
 
         return param_0->absXZ();