Ok, I changed my mind.

The new behavior seems to make more sense to me. Not that it matters,
because we always use nm_utils_strbuf*() API with buffers that we expect
to be large enough to contain the result. And when truncation occurs,
we usually don't care much about it. That is, there is no code that
uses nm_utils_strbuf*() API and handles string truncation in particular.