[RFE] null terminate strings

[grexe]

I think it's an unnecessary trap, doesn't affect performance or memory consumption and it's also uncommon to pass non null terminated strings to a caller in a C library.

For some reason I could not reliably retrieve strings from MD_ATTRIBUTES' text field even when correctly using the size parameter, so I adapted the build_md_attr method and now it works reliably and transparently without unnecessary client checks.

I'd love to share my changes in a PR to this issue if that's okay with you.