xrow: fix crash on nested map/array update ops

If an update operation tried to insert a new key into a map or an
array which was created by a previous update operation, then the
process would fail an assertion.

That was because the first operation was stored as a bar update.
The second operation tried to branch it assuming that the entire
bar update's JSON path must exist, but it wasn't so for the newly
created part of the path.

The solution is to fallback to branching earlier than the entire
bar path ends, if can see that the next part of the path can't be
found.

Closes #7705

NO_DOC=bugfix