[2019-07-31] - Found with Honggfuzz

NULL pointer dereference in the domain_dname() function.

Issue can be reproduced by running:

```
nsd-checkzone all.rr zone04
```

In namedb.h, L270, returning 'domain->dname' results in a NULL pointer
dereference because 'domain' ends up being NULL:

```
#0  0x0000041f7a8f3e2b in domain_dname (domain=0x0) at namedb.h:270
270             return (dname_type *) domain->dname;
```