Update netutils.go

[guoguodan]

add param check，it may be null pointer

[guoguodan]

add param check，it may be null pointer

[codecov-commenter]

Codecov Report

Merging #1561 (39e1e0b) into master (d2a749a) will increase coverage by 1.62%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1561      +/-   ##
==========================================
+ Coverage   49.16%   50.79%   +1.62%     
==========================================
  Files         145      145              
  Lines        8408     9592    +1184     
==========================================
+ Hits         4134     4872     +738     
- Misses       3887     4323     +436     
- Partials      387      397      +10     

Impacted Files	Coverage Δ
pkg/netutils/netutils.go	55.33% <0.00%> (+2.62%)	:arrow_up:
supernode/server/api/api.go	85.18% <0.00%> (-4.29%)	:arrow_down:
dfget/locator/manager.go	69.23% <0.00%> (-3.50%)	:arrow_down:
supernode/daemon/mgr/preheat/base_worker.go	18.60% <0.00%> (-1.99%)	:arrow_down:
dfget/config/supernode_value.go	64.15% <0.00%> (-1.74%)	:arrow_down:
supernode/store/errors.go	73.33% <0.00%> (-1.67%)	:arrow_down:
dfget/core/api/download_api.go	30.23% <0.00%> (-1.35%)	:arrow_down:
dfget/types/pull_piece_task_response.go	85.71% <0.00%> (-1.25%)	:arrow_down:
cmd/dfget/app/root.go	71.34% <0.00%> (-1.03%)	:arrow_down:
supernode/config/config.go	93.75% <0.00%> (-0.99%)	:arrow_down:
... and 136 more

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