Mouse: Don't try to unbind delegated event handlers if they don't exist. Fixes #8416 - Draggable breaks during drag if any other draggable is removed or destroyed.

(cherry picked from commit e68bee9)

Author: scottgonzalez

ui/jquery.ui.mouse.js

@@ -45,9 +45,11 @@ $.widget("ui.mouse", {
 	// other instances of mouse
 	_mouseDestroy: function() {
 		this.element.unbind('.'+this.widgetName);
-		$(document)
-			.unbind('mousemove.'+this.widgetName, this._mouseMoveDelegate)
-			.unbind('mouseup.'+this.widgetName, this._mouseUpDelegate);
+		if ( this._mouseMoveDelegate ) {
+			$(document)
+				.unbind('mousemove.'+this.widgetName, this._mouseMoveDelegate)
+				.unbind('mouseup.'+this.widgetName, this._mouseUpDelegate);
+		}
 	},
 
 	_mouseDown: function(event) {