string is of type char, so a couple of different OSs were complaining.
The 2nd parameter to isc_buffer_putmem() should probably be a const void *,
but rather than deal with changing the API and whatever that implies
with regard to versioning and whatever else, I have instead worked around
the issue by using isc_buffer_putuint8() to add a NUL to the buffer in
the two places that isc_buffer_putmem(&b, "", 1) was used.