Update the code for IPv6 authorities and fallbacks for function
argument changes.
Update unit tests affected by the function argument changes in
the patch.
Add unit tests for authority and fallback:
* adding via a function
* line parsing
* adding default authorities
(Adding default fallbacks is unit tested in #15775.)
The tests pass empty digest strings to the dir_server_new function which
copies it into a directory server structure. The copy operation expects
the digest strings to be DIGEST_LEN characters long.
Because the length of the empty digest strings are lower than
DIGEST_LEN, the copy operation reads outside the digest strings which
leads to buffer over-reads.
The issue is resolved by using character arrays with a size of
DIGEST_LEN.
Patch on 4ff08bb581.
