unsupported screen resolution crashes sdl-qemu

if the (windows) guest sets a screen resolution that the SDL backend does not support,
qemu does an exit(1).
with this fix, the the resolution is still wrong (only part of the desktop is displayed),
but qemu keeps running and the guest can auto-revert the video mode:

ui/sdl.c:do_sdl_resize()
    SDL_Surface * tmp_screen;
    tmp_screen = SDL_SetVideoMode(width, height, bpp, flags);
    if (!tmp_screen) {
//      fprintf(stderr, "Could not open SDL display (%dx%dx%d): %s\n", width, 
//              height, bpp, SDL_GetError());
//      exit(1);
    } else {
        real_screen = tmp_screen;
    }