fix: use last on drag stop position if it's not avalible

If position is not avalible on drag stop, use last position. This fixes
tests for react-grid-layout#621

Author: offlinehacker

lib/DraggableCore.js

@@ -385,6 +385,10 @@ export default class DraggableCore extends React.Component<DraggableCoreProps, D
     if (position == null) return;
     let {x, y} = position;
 
+    // if we couldn't get position from event use lastX and lastY
+    if (isNaN(x)) x = this.state.lastX;
+    if (isNaN(y)) y = this.state.lastY;
+
     // Snap to grid if prop has been provided
     if (Array.isArray(this.props.grid)) {
       let deltaX = x - this.state.lastX, deltaY = y - this.state.lastY;