Don't override user provided sub-branch address.
Don't override user provided sub-branch address.

Prior to this commit doing:
```
// Not setting the top level branch address.
chain->SetBranchAdress(sub_branch_name, &address);
chain->GetEntry(0);
```
Resulted in the address set to be forgotten.  Note that a work-around was:
```
// Not setting the top level branch address.
chain->GetEntry(0);
chain->SetBranchAdress(sub_branch_name, &address);
```
But also the address needed to (in most cases) also be set again after each new tree was loaded.

Note that, the following:
```
chain->SetBranchAdress(sub_branch_name, &address);
chain->SetBranchAdress(top_level_branch_name, &other_address);
chain->GetEntry(0);
```
will result (as one would expect) with the first SetBranchAddress being ignored/over-ridden.

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