SECURITY: Ignore apparent answers before first RR we found the first time

SECURITY: Ignore apparent answers before first RR we found the first time

This way the second answer scan finds the same RRs at the first.
Otherwise, adns can be confused by interleaving answers for the CNAME
target, with the CNAME itself.

In that case the answer data structure (on the heap) can be overrun.

With this change, we prefer to look only at the answer RRs which come
after the CNAME, which is at least arguably correct.

Found by AFL 2.35b.  CVE-2017-9109.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>