Fix undefined `b:after` variable

[valeriangalliat]

This plugin defines a after variable but reads b:after later on which causes crashes in rare occurrences. This PR fixes this.

Note that the opposite patch works too, e.g. leaving those 2 lines alone but replacing if b:after =~? '"' by if after =~? '"' later on. If that makes more sense I'll be happy to update this PR.

Related to https://github.com/vim/vim/pull/8887

Cheers!