fix nil dereference: check that httpServer is set before shutting down

[mmetc]

No description provided.

[codecov-commenter]

Codecov Report

Merging #1893 (686a8b6) into master (fde9640) will decrease coverage by 0.70%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1893      +/-   ##
==========================================
- Coverage   54.02%   53.32%   -0.71%     
==========================================
  Files         148      144       -4     
  Lines       19986    19611     -375     
==========================================
- Hits        10798    10458     -340     
- Misses       7992     8014      +22     
+ Partials     1196     1139      -57     

Flag	Coverage Δ
func-crowdsec	45.40% <ø> (-10.27%)	⬇️
func-cscli	44.97% <ø> (ø)
unit-linux	57.98% <0.00%> (+0.02%)	⬆️
unit-windows	?

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Impacted Files	Coverage Δ
pkg/apiserver/apiserver.go	38.20% <0.00%> (-0.13%)	⬇️
cmd/crowdsec/serve.go	22.34% <0.00%> (-32.58%)	⬇️
cmd/crowdsec/api.go	51.85% <0.00%> (-27.78%)	⬇️
cmd/crowdsec/crowdsec.go	43.97% <0.00%> (-1.58%)	⬇️
cmd/crowdsec/main.go	62.94% <0.00%> (-1.02%)	⬇️
pkg/acquisition/modules/docker/docker.go	60.84% <0.00%> (-0.25%)	⬇️
pkg/acquisition/modules/file/tailline_windows.go
pkg/cwhub/path_separator_windows.go
pkg/csplugin/utils_windows.go
...isition/modules/wineventlog/wineventlog_windows.go
... and 2 more

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