[netinstall] Avoid crash when do groups are available

- m_groups is only set to a non-nullptr value when data is received
   and fully processed,
 - avoid nullptr dereference when paging *back* from a netinstall
   page that hasn't loaded groups data.

FIXES #859

src/modules/netinstall/NetInstallPage.cpp

@@ -113,7 +113,13 @@ NetInstallPage::dataIsHere( QNetworkReply* reply )
 
 QList<PackageTreeItem::ItemData> NetInstallPage::selectedPackages() const
 {
+    if ( m_groups )
         return m_groups->getPackages();
+    else
+    {
+        cDebug() << "WARNING: no netinstall groups are available.";
+        return QList<PackageTreeItem::ItemData>();
+    }
 }
 
 void NetInstallPage::loadGroupList()