QFtpPrivate::_q_piError should check if pending is empty before using it

QFtpPrivate::_q_piError in the file qftp.cpp does not call pending.isEmpty() before calling pending.first(). In the case where there are no items in the pending list, this leads to QList::first triggering a fatal assertion. Other member functions of the QFtpPrivate class (and other classes in qftp.cpp), such as _q_piFinished and _q_startNextCommand, do check that pending is not empty before accessing it.