Null-reference errors when using onComplete on the DragFeature control

[bjornharrtell]

I sometimes get null-reference errors when using onComplete on the DragFeature control. The issue seems that this.feature on the control is null at the moment of the handler beeing called and depends on timing when interacting with the control/feature. My guess is that upFeature gets called before doneDragging in some cases. I've reproduced the error in IE7 and Firefox and it seems to happen more often when dragging the feature with a fast movement.

Proposed patch removes the line nulling the this.feature in upFeature, passing tests.

[bjornharrtell]

crschmidt tipped me about #1196 which described the issue ages ago. Attaching patch there and closing this as dupe.