gh-143050: Correct PyLong_FromString() to use _PyLong_Negate() (#145901)

The long_from_string_base() might return a small integer, when the
_pylong.py is used to do conversion.  Hence, we must be careful here to
not smash it "small int" bit by using the _PyLong_FlipSign().

Co-authored-by: Victor Stinner <vstinner@python.org>