🩹 Fix: Properly handle error of "net.ParseCIDR" in "(*App).handleTrustedProxy"

[leonklingele]

Description

• Do not use empty *net.IPNet in case of an error of net.ParseCIDR
• Add test for ctx.IsProxyTrusted()
• Optimize ctx.IsProxyTrusted() by caching method call

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• For new functionalities I follow the inspiration of the express js framework and built them similar in usage
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation - https://github.com/gofiber/docs for https://docs.gofiber.io/
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• If new dependencies exist, I have checked that they are really necessary and agreed with the maintainers/community (we want to have as few dependencies as possible)
• I tried to make my code as fast as possible with as few allocations as possible
• For new code I have written benchmarks so that they can be analyzed and improved

[leonklingele]

See the individual commits for details.

[li-jin-gou]

I noticed that the title and description of your pr are not the same as most PRs. and you should refer to #2152 or #2188 etc.. and modify the title and description of the PR according to the pull request template.

[ReneWerner87]

@leonklingele can you please add one more test case for this kind of behavior
and next time follow the pull request template form where you work through the checklist

[leonklingele]

Updated, ptal.