Replace occaisional segfault with bad data

Previously, Show of Bits32 would cause segfaults once out of every few
thousand times it was called. Now, a ? is emitted for the odd digit.

This is sub-optimal - better would be to fix it. But in the meantime,
it's better not to segfault.
Merge pull request #928 from david-christiansen/bits-show-segfault

Replace occaisional segfault with bad data