[IO] Fix out of bounds read in TClassEdit
[IO] Fix out of bounds read in TClassEdit

When doing I/O of RVec objects, TClassEdit::STLArgs was accessing
an element one after the end of a static array. asan rightly complains.

This commit fixes https://github.com/root-project/root/issues/7903,
which contains more details.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root