Fix special case for C2x strtol binary constant handling (BZ# 30371)

When the base is 0 or 2 and the first two characters are '0' and 'b',
but the rest are no binary digits.  In this case this is no error,
and strtol must return 0 and ENDPTR points to the 'x' or 'b'.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Florian Weimer <fweimer@redhat.com>