fix StaticFS() get "index out of range" if file not found (#1678)

[vkd]

Fixing #1678

We cannot rewrite c.handlers in handler during iterate over it on c.Next(...)

[codecov]

Codecov Report

Merging #1716 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1716      +/-   ##
=========================================
- Coverage    99.4%   99.4%   -0.01%     
=========================================
  Files          42      42              
  Lines        2029    2027       -2     
=========================================
- Hits         2017    2015       -2     
  Misses          9       9              
  Partials        3       3

Impacted Files	Coverage Δ
routergroup.go	100% <ø> (ø)	⬆️

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[thinkerou]

revert #1663 ?
cc @MetalBreaker

[ilijaivanov]

I don't use Gin anymore. Do whatever you have to.
I personally didn't have an issue with this. Maybe something major got rewritten that slipped past me. Just saying that this prevents a real use case - serving a "file not found" HTML page instead of ugly "404 not found" text.