[bash-completion] Fix endless loop when completion.bash sourced twice

I forgot to add the "not _fzf" check into __fzf_orig_completion, so
invoking it twice would rewrite the _fzf_orig_completion_xxx variables
and then cause an endless loop when completion is requested.

Fixes: ef2c29d5 ("[bash-completion] Optimize __fzf_orig_completion_filter")