Fix: IP length not deterministic

[Loyalsoldier]

The length of IPv4 address in Go standard library could be 16
if it is in IPv6-mapped-IPv4 format.

[codecov-commenter]

Codecov Report

Merging #1267 (97338cb) into master (4a174e8) will decrease coverage by 0.03%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master    #1267      +/-   ##
==========================================
- Coverage   44.76%   44.72%   -0.04%     
==========================================
  Files         485      485              
  Lines       29539    29465      -74     
==========================================
- Hits        13222    13179      -43     
+ Misses      14912    14885      -27     
+ Partials     1405     1401       -4     

Impacted Files	Coverage Δ
app/router/config.go	59.03% <ø> (-6.97%)	⬇️
common/session/session.go	70.58% <ø> (ø)
infra/conf/transport_internet.go	35.92% <ø> (ø)
transport/internet/config.pb.go	16.34% <0.00%> (ø)
app/router/condition_geoip.go	78.18% <77.14%> (-12.00%)	⬇️
transport/internet/http/dialer.go	61.17% <100.00%> (ø)
app/router/config.pb.go	23.79% <0.00%> (+1.16%)	⬆️
transport/internet/websocket/connection.go	20.18% <0.00%> (+3.66%)	⬆️
proxy/freedom/freedom.go	49.00% <0.00%> (+4.00%)	⬆️
common/buf/reader.go	88.23% <0.00%> (+5.88%)	⬆️
... and 1 more

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