Remove unused branch in _make_posargs

`names_with_default` is never `NULL`, even if there are no names with defaults. In that case it points to a structure with `size` zero. Rather than eliminating the branch, we leave it behind with an `assert(0)` in case a future change to the grammar exercises the branch.

Author: mjdominus

Parser/action_helpers.c

@@ -555,7 +555,10 @@ _make_posargs(Parser *p,
  *posargs = _get_names(p, names_with_default);
  }
  else if (plain_names != NULL && names_with_default == NULL) {
- *posargs = plain_names;
+// With the current grammar, we never get here.
+// If that has changed, remove the assert, and test thoroughly.
+assert(0);
+*posargs = plain_names;
  }
  else {
  *posargs = _Py_asdl_arg_seq_new(0, p->arena);