Shell is stuck on 100% cpu if last character is a pipe

Hi.

There seems to be a bug when the plugin is triggered on a prompt with pip as the last character.
For example:
```
$> ls |
``` 
and then triggering `history-substring-search-up` freezes my shell.

Using zsh 5.8.1, antigen and the latest version of this plugin.
This also happens when this is the only loaded plugin.