gh-127165: Disallow embedded NULL characters in _interpreters

[ZeroIntensity]

Currently, null characters in shared namespaces cause truncation of the actual string, because it's eventually passed to PyUnicode_FromString, which uses the strlen instead of the actual size. Then, things break because the string unexpectedly is not in the namespace dictionary. This fixes that by just raising if we find an embedded NULL character in _copy_string_obj_raw.

We could support embedded NULLs by storing the size alongside the name, but this is much simpler (and in practice, nobody is trying to name variables containing \x00).

• Issue: Segfault in invalid concurrent.futures.interpreter.WorkerContext #127165

[picnixz]

Suggested change

	if (size != (Py_ssize_t) strlen(str))
	{
	PyErr_SetString(PyExc_ValueError, "found embedded NULL character");
	return NULL;
	}
	if (size != (Py_ssize_t)strlen(str)) {
	PyErr_SetString(PyExc_ValueError, "found embedded NULL character");
	return NULL;
	}