Check that positions exist before accessing them

include/assimp/Vertex.h

@@ -135,7 +135,9 @@ public:
     /** Extract a particular vertex from a anim mesh and interleave all components */
     explicit Vertex(const aiAnimMesh* msh, unsigned int idx) {
         ai_assert(idx < msh->mNumVertices);
+        if (msh->HasPositions()) {
             position = msh->mVertices[idx];
+        }
 
         if (msh->HasNormals()) {
             normal = msh->mNormals[idx];