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.
