Fix LibsToOso crash when not removing prefixes (AcademySoftwareFoundation#2659)

Node instances were meant to be inserted into an isolated nodegraph,
but weren't due to a merge error during development. This lead to name
collisions when called with --removeNdPrefix false.

Author: chellmuth

source/MaterialXGenOsl/LibsToOso.cpp

@@ -364,7 +364,7 @@ int main(int argc, char* const argv[])
         }
 
         // TODO: Check for the existence/validity of the `Node`?
-        mx::NodePtr node = librariesDoc->addNodeInstance(nodeDef, nodeName);
+        mx::NodePtr node = librariesDocGraph->addNodeInstance(nodeDef, nodeName);
 
         std::string oslShaderName = node->getName();
         oslShaderGen->getSyntax().makeValidName(oslShaderName);