fix(#572, #575): ensure icon overrides apply after refresh

[dsaenztagarro]

Resolves #572 (original issue)
Resolves #575 (reported bug)

What I did

I reapplied the changes from #572.

In addition, I fixed the reported bug with 37078ee.

How to reproduce the bug

I configured locally the plugin so I simulate I "require" the plugin from other plugin as dependency, but without calling "setup".

return {
  'nvim-tree/nvim-web-devicons',
  dir="/Users/dsaenz/Code/nvim-web-devicons",
  dev = true,
  config = function()
    require'nvim-web-devicons'
  end
}

With this, when I open NeoVim I got exactly the same error reported by the user:

Explanation

Because setup was not called, user_icons was not initialized properly. As result, calling apply_user_icons from the auto command was 🧨 .

[alex-courtis]

Thank you.

[alex-courtis]

This is the safest course of action, it maintains existing behaviour.

[alex-courtis]

Many thanks for the quick fix.