dns: improve loop detection in label decompression

dns: improve loop detection in label decompression

Previously the number of allowed pointers within a message is equal to
the data in a tvb (16575 in one example). This is still expensive, so
implement an alternative detection mechanism that looks for a direct
self-loop and limits the total pointers to about 256.

Bug: 13633
Change-Id: I803873e24ab170c7ef0b881d3bdc9dfd4014de97
Link: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=1206
Reviewed-on: https://code.wireshark.org/review/21507
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>