Bugs in dn_expand (XS and PP) on mailformed packages

Binary package hint: libnet-dns-perl

the XS implementation puts the return code of netdns_dn_expand into an
unsigned int instead of an int, so that it never finds out if the
function returned an error (e.g. <0).
The PP implementation goes into and endless loop exhausting the stack on
a mailformed DNS packet, where the string compression causes and endless
loop (e.g. the pointer in www.example.<pointer> points to 'www' again etc).

Both problems have been fixed in the attached diff which also contains a
test for this problem.

This allows remote attackers to cause a denial of service (stack consumption) via a malformed compressed DNS packet with self-referencing pointers, which triggers an infinite loop.