[2019-07-18] - Found with American Fuzzy Lop - CVE-2019-13951

Stack-based buffer overflow in the set_ipv4() function, in zscan_rfc1035.rl.

It can be triggered with the following input:

```
ns1.all.rr.org.         IN      A               10.1.0.529999999999999999999999999999999
```

The issue can be be reproduced by creating a 'zones' directory, putting
the above input in a file within the 'zones' directory, and running:

```
gdnsd -c . checkconf
```

Because no bounds checking is being done in the set_ipv4() function, 'len'
ends up being larger than 16:

```
static void set_ipv4(zscan_t* z, const char* end)
{
    char txt[16];
    unsigned len = end - z->tstart;
    memcpy(txt, z->tstart, len);
```

The parser is attempting to parse malformed IPv4 addresses until there
is no input left. I have been able to get 'len' to reach very large values
when generating malformed IPv4 address strings spawning several gigabytes,
which gdnsd will happily attempt to parse.