Fix condition when `n=NULL`
Fix condition when `n=NULL`

If there's no node with the specified fingerprint, the size of the node
vector will be 0 and, consequently, `n` will still be NULL when access
to `n->parent`.

meganz/sdk,meganz/sdk,meganz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,meganz/sdk,Acidburn0zzz/sdk,Acidburn0zzz/sdk,meganz/sdk,Acidburn0zzz/sdk,meganz/sdk,meganz/sdk