Fix #27847: sequencer scene strip rendering crash, due to use of freed

RenderResult.rect. Sergey did all the work to track this down, I'm just
committing a slighty different fix.

source/blender/blenkernel/intern/image.c

@@ -2277,7 +2277,10 @@ static ImBuf *image_get_render_result(Image *ima, ImageUser *iuser, void **lock_
 	ibuf->x= rres.rectx;
 	ibuf->y= rres.recty;
 	
-	if(ibuf->rect_float!=rectf || rect) /* ensure correct redraw */
+	/* free rect buffer if float buffer changes, so it can be recreated with
+	   the updated result, and also in case we got byte buffer from sequencer,
+	   so we don't keep reference to freed buffer */
+	if(ibuf->rect_float!=rectf || rect || !rectf)
 		imb_freerectImBuf(ibuf);
 
 	if(rect)