Fix byte compile warnings

I got following error when `epc.el` is byte-compiled.

```
In epc:controller-methods-update-buffer:
epc.el:843:34:Warning: assignment to free variable `epc:mngr'

In epc:controller-methods-eval-command:
epc.el:855:30:Warning: reference to free variable `epc:mngr'
epc.el:859:49:Warning: reference to free variable `ret'
```