2.5: fix crash loading saved file with render result, image user

scene wasn't set to NULL on load. Still doesn't seem entirely safe
but prevents crash here.

source/blender/blenloader/intern/readfile.c

@@ -4655,6 +4655,7 @@ static void direct_link_screen(FileData *fd, bScreen *sc)
 					direct_link_gpencil(fd, sima->gpd);
 				if(sima->cumap)
 					direct_link_curvemapping(fd, sima->cumap);
+				sima->iuser.scene= NULL;
 				sima->iuser.ok= 1;
 			}
 			else if(sl->spacetype==SPACE_NODE) {