IECoreArnold::MeshAlgo : Fix iterator invalidation bug.
IECoreArnold::MeshAlgo : Fix iterator invalidation bug.

I found this while debugging something else with valgrind. While it definitely fixes a genuine memory access error, unfortunately it doesn't seem to have been the culprit for my particular problem.

appleseedhq/cortex,appleseedhq/cortex,hradec/cortex,hradec/cortex,hradec/cortex,appleseedhq/cortex