SDL2 UI sends a NULL to sdl_grab_start if fullscreen, which crashes

in ui/sdl2.c:

    if (full_screen) {
        gui_fullscreen = 1;
        sdl_grab_start(0);
    }

Is sent, but no null checks are made in sdl_grab_start (its assumed to be an allocated pointer). So a crash happens if you start qemu -full-screen.

It should at lease send the first [0] of the newly allocated sdl2_console through.

Quickly looking around should look something like:

    if (full_screen) {
        gui_fullscreen = 1;
        sdl_grab_start(&sdl2_console[0]);
    }

The NULL pointer check has been added here:
http://git.qemu.org/?p=qemu.git;a=commitdiff;h=f2335791fd0ceb2f9e3