Fix vtkImageSlice Render when mapper doesn't have an input

* Fix SEGFAULT when vtkImageSlice doesn't have an input

Documentation/release/dev/vtkImageSlice-Fix-Render.md

+## Fix `vtkImageSlice` Render when mapper doesn't have an input
+
+Fix `vtkImageSlice` Render which would SEGFAULT when input was empty.

Rendering/Core/vtkImageSlice.cxx

@@ -336,11 +336,14 @@ void vtkImageSlice::Render(vtkRenderer* ren)
 
   // only call the mapper if it has an input
   vtkImageData* input = this->Mapper->GetInput();
-  int* extent = input->GetExtent();
-  if (input && extent[0] <= extent[1] && extent[2] <= extent[3] && extent[4] <= extent[5])
+  if (input)
   {
-    this->Mapper->Render(ren, this);
-    this->EstimatedRenderTime += this->Mapper->GetTimeToDraw();
+    int* extent = input->GetExtent();
+    if (extent[0] <= extent[1] && extent[2] <= extent[3] && extent[4] <= extent[5])
+    {
+      this->Mapper->Render(ren, this);
+      this->EstimatedRenderTime += this->Mapper->GetTimeToDraw();
+    }
   }
 
   vtkImageToImageMapper3DFriendship::SetCurrentRenderer(this->Mapper, nullptr);