Avoid panic when max_resp_code is zero

[simonborje]

This change avoids a division by zero panic when receiving an MLDv2 query message with max_resp_code set to 0.

Fixes #1046

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.17%. Comparing base (3988338) to head (d3ca540).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/iface/interface/multicast.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1047      +/-   ##
==========================================
- Coverage   81.17%   81.17%   -0.01%     
==========================================
  Files          81       81              
  Lines       28958    28956       -2     
==========================================
- Hits        23508    23505       -3     
- Misses       5450     5451       +1     

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