After adding edges in Ng_STL_InitSTLGeometry, set readedges size to 0.

[nikhilsrajan]

The global variable readedges that the API uses doesn't clear itself after the Ng_STL_Geometry * has been initialized. It would cause problems for people who wish to initialize another Ng_STL_Geometry if they used Ng_STL_AddEdge in before their previous initialization.