C2x strtol binary constant handling: Fix special case "0b"

[Bruno Haible]

Hi Joseph,

While adding C2x support to the strtol, strtoll, strtoul, strtoull
functions in Gnulib, I was glad to see that you have already done
the support in glibc. But I noticed that the glibc implementation
apparently does not handle the input "0b" correctly. The unit test
cases for this special case are:

  {
    const char input[] = "0b";
    char *ptr;
    long result;
    errno = 0;
    result = strtol (input, &ptr, 2);
    assert (result == 0L);
    assert (ptr == input + 1);
    assert (errno == 0);
  }
  {
    const char input[] = "0b";
    char *ptr;
    long result;
    errno = 0;
    result = strtol (input, &ptr, 0);
    assert (result == 0L);
    assert (ptr == input + 1);
    assert (errno == 0);
  }

and likewise for strtoll, strtoul, strtoull.

Find attached a correction for this. I have tested it in Gnulib (which
has very similar source code as glibc for this functionality), not in
Glibc directly. Therefore, please review thoroughly.

Bruno
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