DownloadRegionDialog: Fix crash when switching to a map theme without texture layer.
DownloadRegionDialog: Fix crash when switching to a map theme without texture layer.

The problem was that d->m_textureLayer is used in the tiles count calculation. If
there is no texture layer, the tiles count is simply 0.

svn path=/trunk/KDE/kdeedu/marble/; revision=1125660

tzapzoor/marble,utkuaydin/marble,David-Gil/marble-dev,probonopd/marble,quannt24/marble,tzapzoor/marble,probonopd/marble,rku/marble,adraghici/marble,utkuaydin/marble,quannt24/marble,utkuaydin/marble,probonopd/marble,oberluz/marble,rku/marble,David-Gil/marble-dev,tzapzoor/marble,utkuaydin/marble,quannt24/marble,tzapzoor/marble,AndreiDuma/marble,tucnak/marble,Earthwings/marble,David-Gil/marble-dev,rku/marble,quannt24/marble,Earthwings/marble,David-Gil/marble-dev,AndreiDuma/marble,probonopd/marble,rku/marble,utkuaydin/marble,adraghici/marble,utkuaydin/marble,AndreiDuma/marble,adraghici/marble,quannt24/marble,adraghici/marble,Earthwings/marble,quannt24/marble,AndreiDuma/marble,oberluz/marble,tzapzoor/marble,tucnak/marble,David-Gil/marble-dev,oberluz/marble,adraghici/marble,oberluz/marble,adraghici/marble,probonopd/marble,rku/marble,tzapzoor/marble,tucnak/marble,Earthwings/marble,probonopd/marble,David-Gil/marble-dev,tucnak/marble,AndreiDuma/marble,tucnak/marble,oberluz/marble,Earthwings/marble,tucnak/marble,tucnak/marble,AndreiDuma/marble,probonopd/marble,tzapzoor/marble,rku/marble,oberluz/marble,quannt24/marble,tzapzoor/marble,Earthwings/marble