First part of bugfix for #5933

First part of bugfix for #5933

Ofer Tal <otsmb@shmoop.org> fully correctly noted that we're incrementing
num_validated_vuids twice per session setup, but decrement it only once.
Looking at sesssetup.c we always call register_initial_vuid() before
register_existing_vuid(), so there's no point in incrementing it in
register_existing_vuid().

Jeremy, please check!