Fix UploadStash::removeFile() error

If nothing has called getFile() for a key before that key is passed to
removeFileNoAuth(), UploadStash will throw a fatal error when it tries to
remove the file from the filesystem because the UploadStashFile was
never created.

Change-Id: Idf0d2c53ce759b59836f34ff5aaad393b8c371c6