Fix a race condition with interned strings sweeping.

	[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.