core: an empty App segfaults (potential to improve safety checks)

[prime31]

While setting up mach in a project I discovered an empty App will result in a segfault.

Full repro of main.zig:

pub const App = struct {
    pub fn init(_: *App) !void {}

    pub fn update(_: *App) !bool {
        return false;
    }

    pub fn deinit(_: *App) void {}
};