xrow: fix bar new field update crash

A tuple update with the first operation creating a new field
somewhere deep in the tuple and the second operation trying to go
into that new field could crash. This happened because the route
branching function xrow_update_route_branch() missed this case.

It can be detected when see that the bar path is already fully
used (the next JSON token is END), and the new operation's path
is still not END.

Closes #8216

NO_DOC=bugfix