read rest of buffer when any errors

[beyondkmp]

当outbound遇到错误时，close pipe, 这样可以使pipe里面的数据读取完。如果直接closeerror的话，会清空整个pipe里面的内容，这样有点不太合理。标准库里面的read接口也是读取了多少数据就输出多少。fix #1082

[codecov]

Codecov Report

Merging #1089 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1089      +/-   ##
==========================================
+ Coverage   75.33%   75.39%   +0.06%     
==========================================
  Files         229      229              
  Lines       10074    10072       -2     
==========================================
+ Hits         7589     7594       +5     
+ Misses       1765     1755      -10     
- Partials      720      723       +3

Impacted Files	Coverage Δ
app/proxyman/outbound/handler.go	70.14% <100%> (-0.87%)	⬇️
common/stats/io.go	71.42% <0%> (-28.58%)	⬇️
common/log/logger.go	84.37% <0%> (-3.13%)	⬇️
transport/internet/kcp/sending.go	87.11% <0%> (-1.34%)	⬇️
proxy/vmess/inbound/inbound.go	77.24% <0%> (+0.52%)	⬆️
app/proxyman/mux/mux.go	52.87% <0%> (+1.43%)	⬆️
app/proxyman/mux/session.go	83.56% <0%> (+12.32%)	⬆️

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[DarienRaymond]

Already fixed.