Previously, dns_answer_add() was O(n^2).
With this change dns_packet_extract() becomes ~15 times faster for some
extremal case.
Before:
```
$ time ./fuzz-dns-packet ~/downloads/clusterfuzz-testcase-minimized-fuzz-dns-packet-5631106733047808
/home/watanabe/downloads/clusterfuzz-testcase-minimized-fuzz-dns-packet-5631106733047808... ok
real 0m15.453s
user 0m15.430s
sys 0m0.007s
```
After:
```
$ time ./fuzz-dns-packet ~/downloads/clusterfuzz-testcase-minimized-fuzz-dns-packet-5631106733047808
/home/watanabe/downloads/clusterfuzz-testcase-minimized-fuzz-dns-packet-5631106733047808... ok
real 0m0.831s
user 0m0.824s
sys 0m0.006s
```
Hopefully fixes oss-fuzz#19227.
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19227
Yu Watanabe