fix error of panic

[jkralik]

No description provided.

[codecov-commenter]

Codecov Report

Merging #346 (d85b34a) into master (8a6df8e) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
+ Coverage   69.46%   69.60%   +0.14%     
==========================================
  Files          67       67              
  Lines        5669     5669              
==========================================
+ Hits         3938     3946       +8     
+ Misses       1322     1315       -7     
+ Partials      409      408       -1     

Impacted Files	Coverage Δ
udp/server.go	84.00% <100.00%> (+1.77%)	⬆️
net/conn.go	85.93% <0.00%> (+1.56%)	⬆️
dtls/server.go	85.71% <0.00%> (+1.71%)	⬆️

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