Only compute user/repo if suggesting badges for GitHub

[espadrine]

This resolves the following error occurring in production:

TypeError: Cannot read property 'slice' of null
/home/m/shields/lib/suggest.js in findSuggestions at line 40:33
  const userRepo = url.pathname.slice(1).split('/');

[shields-ci]

	Warnings
⚠️	This PR modified helper functions in lib/ but not accompanying tests. That's okay so long as it's refactoring existing code.

	Messages
📖	✨ Thanks for your contribution to Shields, @espadrine!

Generated by 🚫 dangerJS

[platan]

@paulmelnikow @espadrine is this reported by Sentry? What do you think about labeling all bugs/problems reported by Sentry? This would show us how helpful Sentry is in our case.

[paulmelnikow]

Yes! That sounds great.

There are a handful of open issues in Sentry and while they don’t take very long to fix, I was thinking if I open an issue for each one, I can paste in stack traces or screenshots, and we can spread the work around.

Would also be good to include tests with the future fixes!