fix ObNumber desc_ to adjust digits' len

[raygift]

What changes were proposed in this pull request?

As ObNumber.len_ should be same with digits' len, change desc_ of ObNumber constructor from 3 to 1

Why are the changes needed?

issue https://github.com/oceanbase/oceanbase/issues/1068

Will break the compatibility? How if so?

Does this PR introduce any user-facing change?

No

How was this patch tested?

unit test test_row_reader

Checklist

• [x] I've run the tests to see all new and existing tests pass.
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above.
• [ ] I've informed the technical writer about the documentation change if necessary.