Fixed two bugs in GUI

* Fixed out-of bounds warnings, when showing 2D-Textures in Inspector
* Fixed bug introduced in 719004e0 which would crash the application when the
  PipelineTreeWidget had no item selected (e.g. when adding a new Pipeline)

application/gui/datacontainerinspectorcanvas.cpp

@@ -431,7 +431,7 @@ namespace campvis {
         }
 
         if (maxSlices == 1)
-            maxSlices = -1;
+            maxSlices = 0;
         p_currentSlice.setMaxValue(maxSlices - 1);
         _texturesDirty = false;
         _geometriesDirty = false;

application/gui/pipelinetreewidget.cpp

@@ -382,7 +382,8 @@ namespace campvis {
     void PipelineTreeWidget::selectionChanged(const QItemSelection& selected, const QItemSelection &deselected) {
         QTreeView::selectionChanged(selected, deselected);
         QModelIndexList selection = this->selectedIndexes();
-        emit itemChanged(selection.first());
+        if (!selection.isEmpty())
+            emit itemChanged(selection.first());
     }
 
     void PipelineTreeWidget::setupWidget() {