[2019-07-31] - Found with Honggfuzz

NULL pointer dereference in the rdata_atom_size() function.

Issue can be reproduced by running:

```
nsd-checkzone all.rr zone03
```

In namedb.h, L347, returning '*atom.data' results in a NULL pointer
dereference because 'atom.data' ends up being NULL:

```
#0  0x00000dd0a0deb360 in rdata_atom_size (atom={domain = 0x0, data = 0x0}) at namedb.h:347
347             return *atom.data;
```