Bugfix: When using 'border render' (part image), and then in compositor

the Preview Panel, you got a crash... this case wasn't anticipated in
code... two cropping mechanisms on top. :)

Works OK now, but offset of cropping is drawn a bit weird... this case
needs recode a bit. On list for later.

source/blender/blenkernel/intern/node_composite.c

@@ -459,7 +459,7 @@ static void generate_preview(bNode *node, CompBuf *stackbuf)
 {
 	bNodePreview *preview= node->preview;
 	
-	if(preview) {
+	if(preview && stackbuf) {
 		CompBuf *cbuf;
 		
 		if(stackbuf->x > stackbuf->y) {

source/blender/src/drawimage.c

@@ -1057,6 +1057,11 @@ static void preview_cb(struct ScrArea *sa, struct uiBlock *block)
 	int winy= (G.scene->r.size*G.scene->r.ysch)/100;
 	short mval[2];
 	
+	if(G.scene->r.mode & R_BORDER) {
+		winx*= (G.scene->r.border.xmax - G.scene->r.border.xmin);
+		winy*= (G.scene->r.border.ymax - G.scene->r.border.ymin);
+	}
+	
 	/* while dragging we don't update the rects */
 	if(block->panel->flag & PNL_SELECT)
 		return;