SWIG's "DOH String" can hold a string containing zero bytes, but SWIG is a bit careless in handling literal string values currently and they can get truncated at the zero byte.

This is a fairly uncommon case, but I think it's also pretty close to just working. Also #2990 broke some cases which previously worked (in exchange for fixing some more common cases which were previously mishandled) and it would be good not to regress these. I'll try to do this for 4.3.0.