fixed node.end_of_math

git-svn-id: https://foundry.supelec.fr/svn/luatex/branches/experimental@4974 0b2b3880-5936-4365-a048-eb17d2e5a6bf
some experimental code and some fix proposed

git-svn-id: https://foundry.supelec.fr/svn/luatex/branches/experimental@4975 0b2b3880-5936-4365-a048-eb17d2e5a6bf