knot_dname_cmp is not comparing null octets correctly
This name `hello\000.dev.` is greater than `test.hello.dev.` according to canonical DNS Name Order (RFC4034 section 6.1 ).
```c
knot_dname_t *d1 = knot_dname_from_str_alloc("test.hello.dev.");
knot_dname_t *d2 = knot_dname_from_str_alloc("hello\\000.dev.");
printf("%d\n", knot_dname_cmp(d1, d2)); // output: 116
// doesn't happen with \001
d2 = knot_dname_from_str_alloc("hello\\001.dev.");
printf("%d\n", knot_dname_cmp(d1, d2)); // output: -1
```
This was causing issues with NSEC validation in knot resolver which I mentioned on gitter chat.
issue