[RF] Convert old-style for loop to STL style in RooProduct.
[RF] Convert old-style for loop to STL style in RooProduct.

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