[DF] Do not call SetBranchAddress when not needed
[DF] Do not call SetBranchAddress when not needed

This removes spurious warnings from dataframe_snapshot.cxx.
The problem (once again) was that map::operator[] creates entries
if not present.

olifre/root,karies/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root