fix dequeuing of probe packets

[marten-seemann]

This is analogous to #1858.

[codecov-io]

Codecov Report

Merging #1876 into gquic will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            gquic    #1876      +/-   ##
==========================================
+ Coverage   89.13%   89.13%   +<.01%     
==========================================
  Files         124      124              
  Lines        8842     8844       +2     
==========================================
+ Hits         7881     7883       +2     
  Misses        651      651              
  Partials      310      310

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_handler.go	80.4% <100%> (+0.11%)	⬆️

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