Fix incorrect UDP length in Torjan Packet Addr

[xiaokangwang]

This pull request fix a bug introduced in #2225 that result in a broken UDP implementation in Trojan protocol when Packet Addr is used.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c8b6237) 39.03% compared to head (9308de4) 39.04%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2424   +/-   ##
=======================================
  Coverage   39.03%   39.04%           
=======================================
  Files         617      617           
  Lines       37035    37036    +1     
=======================================
+ Hits        14458    14460    +2     
+ Misses      20983    20982    -1     
  Partials     1594     1594           

Impacted Files	Coverage Δ
app/dns/dns.go	63.44% <0.00%> (-0.22%)	⬇️
proxy/trojan/client.go	1.02% <0.00%> (ø)

... and 4 files with indirect coverage changes

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