DNS decompression loops

[gaya-cohen]

Parsing malformed DNS packets places the library in a while-true state.
Attached is a capture file.

loop_each_other_responses.cap.gz

Notice the function DNS::compose_name in dns.cpp only contains a check for the offset not being too low or high, but not a check for whether the offset loops.

#444

[mfontanini]

Closing as #444 fixed it. Thanks for raising the issue and PR!