Fix T40653: Modified keymap: MMB not behaving like RMB?

Modal frame_change was not terminating on mmb release event...

source/blender/editors/animation/anim_ops.c

@@ -168,6 +168,7 @@ static int change_frame_modal(bContext *C, wmOperator *op, const wmEvent *event)
 		
 		case LEFTMOUSE: 
 		case RIGHTMOUSE:
+		case MIDDLEMOUSE:
 			/* we check for either mouse-button to end, as checking for ACTIONMOUSE (which is used to init 
 			 * the modal op) doesn't work for some reason
 			 */