Fix max client name length in HiddenServiceAuthorizeClient description

REND_CLIENTNAME_MAX_LEN is set to 16, not 19.
tweak changes file.
add test for tor_addr_make_null

Signed-off-by: Kevin Murray <spam@kdmurray.id.au>
Fix compilation warnings in tor_addr_make_null patch

There was one "missing prototype" warning because the test function
wasn't static, and one "unused parameter" warning about the "data"
parameter.

Also, I added a couple of tests to make sure that the "make_null"
addresses really were the addresses we expected, by formatting them
as strings.