MaterialX Null Pointer Dereference in getShaderNodes due to Unchecked nodeGraph->getOutput return

SummaryWhen parsing shader nodes in a MTLX file, the MaterialXCore code accesses a potentially null pointer, which can lead to crashes with maliciously crafted files. DetailsIn src/MaterialXCore/Material.cpp, in function getShaderNodes, the following code fetches the output nodes for a given...