Fix issue reported on irc by Sebastian, crash when undoing and changing

screen.

Apparently the screen on the given file did not have a scene attached.

Not sure how this is possible exactly, but for now just guard against it
at load time by assigning default scene in that case.

source/blender/blenloader/intern/readfile.c

@@ -5838,6 +5838,12 @@ static void lib_link_screen(FileData *fd, Main *main)
 		if (sc->id.flag & LIB_NEED_LINK) {
 			sc->id.us = 1;
 			sc->scene = newlibadr(fd, sc->id.lib, sc->scene);
+
+			/* this should not happen, but apparently it does somehow. Until we figure out the cause,
+			 * just assign first available scene */
+			if (!sc->scene)
+				sc->scene = main->scene.first;
+
 			sc->animtimer = NULL; /* saved in rare cases */
 			
 			for (sa = sc->areabase.first; sa; sa = sa->next) {