Eliminate the crash on Unix caused by the QLockFile usage

We should not check against null because swap should always take place.
Initially, we do not construct a valid pointer guarded by the QScopedPointer
because not every constructor takes the portname argument, so it is possible to
create a QSerialPort instance without portname, and set it later.

Hence, removing the check makes sense, otherwise the smart pointer will guard a
null raw pointer which will cause crash all of a sudden when trying to be used.

Change-Id: Ib383f6eef2761ca5822585b848b0d29cce4eeb45
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>