From Farshid Lashkari, "The Collada loader would crash while processing textures on certain files. I've attached the fix."

git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/branches/OpenSceneGraph-3.2@14596 16af8721-9629-0410-8352-f15c8da7e697

src/osgPlugins/dae/daeRMaterials.cpp

@@ -1047,7 +1047,7 @@ osg::Texture2D* daeReader::processTexture(
             return NULL;
         }
 
-        if (sampler->getSource() == NULL )
+        if (sampler->getSource() == NULL || sampler->getSource()->getValue() == NULL)
         {
             OSG_WARN << "Could not locate source for sampler2D" << std::endl;
             return NULL;