extra: handle an edge case in BigUint.to_str().

If any of the digits was one past the maximum (e.g. 10**9 for base 10),
then this wasn't detected correctly and so the length of the digit was
one more than expected, causing a very large allocation.

Fixes #10522.
Fixes #10288.
auto merge of #10523 : huonw/rust/10522, r=cmr

If any of the digits was one past the maximum (e.g. 10**9 for base 10),
then this wasn't detected correctly and so the length of the digit was
one more than expected, causing a very large allocation.

Fixes #10522.
Fixes #10288.