Bug 998175 - Properly handle concurrent file transfers

Shell we have a check first `if (this._sendingFilesQueue[0].viaHandover)` since the property `viaHandover` is usually  to be undefined here. It will make error console occurred if we access an undefined property.

`if (this._sendingFilesQueue.length > 0 && this._sendingFilesQueue[0].viaHandover)`