MaterialX's unchecked nodeGraph->getOutput return is vulnerable to NULL Pointer Dereference

MaterialX is an open standard for the exchange of rich material and look-development content across applications and renderers. In version 1.39.2, when parsing shader nodes in a MTLX file, the MaterialXCore code accesses a potentially null pointer, which can lead to crashes with maliciously crafted files. An attacker could intentionally crash a target program that uses OpenEXR by sending a malicious MTLX file. This is fixed in version 1.39.3.