Fix for bug printing BigFloat where output fills internal DIGITS buffer

Internals & Design
1

I have a fix for the bug that @TotalVerb reported, https://github.com/JuliaLang/julia/issues/22126, with a unit test added,
in https://github.com/ScottPJones/julia/tree/spj/fixprintf, if somebody wants to pull it in (@tkelman?)

2

Thanks @vtjnash for getting this in! (https://github.com/JuliaLang/julia/pull/22154)