MySQL Bug 86209 Overflow in digest_add_token

Before this fix, some queries could cause
an overflow in the digest code,
in function digest_add_token().

External contribution from:
- Laurynas Biveinis
- Roel van de Paar

Please see the original contribution
attached to the bug report for:
- the root cause analysis
- the test cases
- the rationale for the fix
- the full change set comments.