Fix double free caused in FindInvalidDataProcess

In debug builds, there's no issues, but in release, an invalid mesh that
is deleted in FindInvalidDataProcess will be double free'd when the
parent scene is destroyed.

code/PostProcessing/FindInvalidDataProcess.cpp

@@ -124,7 +124,7 @@ void FindInvalidDataProcess::Execute(aiScene *pScene) {
         if (2 == result) {
             // remove this mesh
             delete pScene->mMeshes[a];
-            AI_DEBUG_INVALIDATE_PTR(pScene->mMeshes[a]);
+            pScene->mMeshes[a] = NULL;
 
             meshMapping[a] = UINT_MAX;
             continue;