Fix handling of unsigned ntp deny messages.

[davidv1992]

Fixes #1867

[codecov]

Codecov Report

Attention: Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.61%. Comparing base (cc70508) to head (802fb4c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
ntp-proto/src/source.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1874      +/-   ##
==========================================
+ Coverage   81.58%   81.61%   +0.02%     
==========================================
  Files          67       67              
  Lines       21998    22024      +26     
==========================================
+ Hits        17948    17975      +27     
+ Misses       4050     4049       -1     

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