src/addrfam.c (adns_addr2text): Don't print junk in debugging output.

At the point we print `adns_addr2text: will print scoped addr ...' the
buffer isn't null-terminated.  Fortunately, we have the length of the
valid portion of the buffer, so use that to print only the good piece.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>