Fix possible correctness bug in VectorTree.
Fix possible correctness bug in VectorTree.

After erase(), the iterator to use it is invalid. Use
the new location it returns for the subsequent insert().

Signed-off-by: Henner Zeller <259ad50fabbfd39355f36ecd6c0afca541ea66ef@acm.org>

chipsalliance/verible,chipsalliance/verible,chipsalliance/verible,chipsalliance/verible,chipsalliance/verible