Fix nullptr crash in the OpenEXR Python module.

PyErr_NewException() returns null at runtime so guard against it.
It's unclear why the function returns null but guarding against it
doesn't cause any other known issues aside from the loss of the
old `OpenEXR.error` exception class.

Author: rvinluan-sidefx

src/wrappers/python/PyOpenEXR_old.cpp

@@ -1572,8 +1572,11 @@ init_OpenEXR_old(PyObject* module)
     PyModule_AddObject (module, "OutputFile", (PyObject*) &OutputFile_Type);
 
     OpenEXR_error = PyErr_NewException ((char*) "OpenEXR.error", NULL, NULL);
-    PyDict_SetItemString (moduleDict, "error", OpenEXR_error);
-    Py_DECREF (OpenEXR_error);
+    if (OpenEXR_error)
+    {
+	PyDict_SetItemString (moduleDict, "error", OpenEXR_error);
+	Py_DECREF (OpenEXR_error);
+    }
 
     PyObject *item;