DNS AM: add a bit of defensive programming

[p-l-]

This PR adds some checks in the DNS answering machine code that parses the received packets.

Checklist:

• [x] I squashed commits belonging together
• [x] I added unit tests or explained why they are not relevant
• [ ] I executed the regression tests (using cd test && ./run_tests or tox)

fixes #4090

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 82.17%. Comparing base (01aeed2) to head (32ef3af).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4092   +/-   ##
=======================================
  Coverage   82.17%   82.17%           
=======================================
  Files         350      350           
  Lines       83261    83282   +21     
=======================================
+ Hits        68419    68441   +22     
+ Misses      14842    14841    -1     

Files	Coverage Δ
scapy/layers/dns.py	88.62% <87.50%> (-0.10%)	:arrow_down:

... and 5 files with indirect coverage changes