Fix case that input regexp matches empty string

searching for \| will go into an infinite loop · Issue #29 · ShingoFukuyama/helm-swoop · GitHub

Input regexp which matches empty string(zero width match) has
caused infinite loops and hungs up.

I fixed this issue. We can use more complex regexp like 'OR',
'word bound', 'symbol bound' etc and use helm-swoop more useful.

I can find such bug because I fixed such bugs in emacs-anzu. :)