BUG/MINOR: uri-auth: avoid leaks on initialization error
When stats_add_scope() and stats_add_auth() fail to initialize a field,
they just leave a partially allocated and initialized structure behind
them that is leaked. The whole architecture doesn't provide clean
unrolling abilities since everything is shared and assigned unconditionally,
but let's at least release what was just allocated. The whole approach would
probably deserve being revisited if one day this becomes more dynamic.