Fix potential BeforeUpload crashes

If destination is chosen and becomes invalid, BeforeUpload crashed e.g.
https://s3-ap-southeast-2.amazonaws.com/mcored/ShareX/2014-07/ShareX.vshost-ShareX_9.3_Beta_2014-07-30_06-26-00.png
and
https://s3-ap-southeast-2.amazonaws.com/mcored/ShareX/2014-07/ShareX.vshost-ShareX_-_Error_2014-07-30_05-54-35.png