fzf: fix vim plugin patch phase

During patch phase, the path to the fzf binary is overwritten in the vim
plugin source. The sed expression doing this wasn't working because the fzf
code changed in this commit:
https://github.com/junegunn/fzf/commit/02ceae15a235de6b5cd2aca9de070fb3fff78e5b
making the patch useless.

I fixed it and added a check to verify that the plugin was effecively
patched to prevent this to happen in the future.