wayland: Fix crash when wayland server is not running

Fix two ```NULL``` pointer dereferencing crashes in ```Wayland_VideoQuit```.

How to reproduce the bug (and verify it is fixed with this patch):

* Build SDL2 with ```--enable-video-wayland --disable-video-x11```
* ```export SDL_VIDEODRIVER=wayland```
* Make sure no wayland server is running (and unset ```WAYLAND_DISPLAY```)
* Build this simple test application:

```
#include <SDL2/SDL.h>

int
main(int argc, char *argv[])
{
    SDL_Init(SDL_INIT_VIDEO);
    SDL_Quit();
    return 0;
}
```

* Without this patch, the test application crashes:

```
#0  0x00000031e5c04fc9 in wl_proxy_destroy ()
   from /lib64/libwayland-client.so.0
#1  0x00007ffff7dcc64e in wl_output_destroy (wl_output=<optimized out>)
    at /usr/include/wayland-client-protocol.h:2107
#2  Wayland_VideoQuit (_this=0x6020c0)
    at ../src/video/wayland/SDL_waylandvideo.c:279
#3  0x00007ffff7dc387e in SDL_VideoQuit () at ../src/video/SDL_video.c:2227
#4  0x00007ffff7dc3e73 in SDL_VideoInit (driver_name=<optimized out>,
    driver_name@entry=0x0) at ../src/video/SDL_video.c:517
#5  0x00007ffff7d2cecf in SDL_InitSubSystem (flags=flags@entry=32)
    at ../src/SDL.c:121
#6  0x00007ffff7d2cf5c in SDL_Init (flags=32) at ../src/SDL.c:216
#7  0x0000000000400709 in main ()
```

Tested with wayland 1.1.90-0.1.20130515 on Fedora 19.