BUG/MEDIUM: dns: fix accepted_payload_size parser to avoid integer overflow

BUG/MEDIUM: dns: fix accepted_payload_size parser to avoid integer overflow

Since commit 9d8dbbc ("MINOR: dns: Maximum DNS udp payload set to 8192") it's
possible to specify a packet size, but passing too large a size or a negative
size is not detected and results in memset() being performed over a 2GB+ area
upon receipt of the first DNS response, causing runtime crashes.

We now check that the size is not smaller than the smallest packet which is
the DNS header size (12 bytes).

No backport is needed.