[Bug #21172]

This fixes a rare CI failure.

The timeline of the race condition is:

- A `"foo" oid=1` string is interned.
- `"foo" oid=1` is no longer referenced and will be swept in the future.
- Another `"foo" oid=2` string is interned.
- `register_fstring` finds `"foo" oid=1`, but since it is about to be swept,
  removes it from `fstring_table` and insert `"foo" oid=2` instead.
- `"foo" oid=1` is swept, since it has the `RSTRING_FSTR` flag,
  a `st_delete` is issued in `fstring_table` which removes `"foo" oid=2`.

I don't know how to reproduce this bug consistently in a single test
case.