From Mikhail Izmestev, "Attached fix to avoid vector usage in StateGraph::prune and reduce heap allocations."

Notes from Robert Osfield, ammended the erase so that it explictly increments the iterator before the erase call.


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14277 16af8721-9629-0410-8352-f15c8da7e697