BGE: fix [#35447] LibLoad Meshes and Scenes causes crash of blender

KX_BlenderSceneConverter's destructor freed the contents of several std::vectors without removing those contents from the vectors. Later these vectors are iterated through. Making sure the vectors get cleared solves this problem.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57596 954f8c5b-7b00-dc11-b283-0030488c597c