powerdns-recursor: denial of service

A bug was discovered in our label decompression code, making it possible
for names to refer to themselves, thus causing a loop during
decompression. This loop is capped at a 1000 iterations by a failsafe,
making the issue harmless on most platforms.
However, on specific platforms, the recursion involved in these 1000
steps causes memory corruption leading to a quick crash, presumably
because the default stack is too small.