"Robustify" the RTC buffer getters (log, ptx, lto_ir)

[eyalroz]

Using the opportunity of our API for buffer getters in the RTC program output changing from the not-really-dynarray to a span, let's also make sure all of the actual-text-strings there are put in buffers with a trailing '\0'. This is not strictly necessary, as a span is not guaranteed to also work as a C-style, nul-terminated string - but it's safer if we do that, in case someone mishandles the returned container and treats its data like that.