[BUGFIX] Only try to move uploaded file if temp file exists

Change-Id: I762072f058d0b6a1fd704e20b31c0e0aad5081d2
Reviewed-on: http://review.typo3.org/33026
Reviewed-by: Sebastian Fischer <typo3@evoweb.de>
Tested-by: Sebastian Fischer <typo3@evoweb.de>