Fixed bug 1923 - Crash with SDL_SetColorKey

Sylvain

1/ Load an Image XPM with IMG_ReadXPMFromArray()
2/ Try to set a ColorKey on it.

I notice that :
- the SDL_Surface that is created from XPM has a palette !
- the colorkey is a RBG.

it crashes (SIGSEGV) inside the SDL_SetColorKey function:
"surface->format->palette->colors[surface->map->info.colorkey].a"