Fixed analyzer warning about possible null deref

Added check for null, other code above was doing it.

Change-Id: I0f58a1957d4586801b98805e0bd9bdcdf115eeb2

Rendering/OpenGL/vtkOpenGLImageSliceMapper.cxx

@@ -323,7 +323,8 @@ void vtkOpenGLImageSliceMapper::RenderTexturedPolygon(
       }
 
     GLenum interp = GL_LINEAR;
-    if (property->GetInterpolationType() == VTK_NEAREST_INTERPOLATION &&
+    if (property &&
+        property->GetInterpolationType() == VTK_NEAREST_INTERPOLATION &&
         !this->ExactPixelMatch)
       {
       interp = GL_NEAREST;