Ordering by ambiguous column crashed vtgate

[systay]

On the latest Vitess 19.0 release, a simple query like:

select foobar from user u join music m on u.id = m.user_id order by foobar

will fail during semantic analysis with a panic:

Uncaught panic:
runtime error: index out of range [-1]
...
vitess.io/vitess/go/vt/vtgate/semantics/early_rewriter.go:671
...

The tables are not authoritative in the vschema, and the two sides of the join can be merged thanks to sharding keys.
The issue is no longer present in main

[systay]

Fixed by #17103