tests: add a test case for ofproto_protocol
ofproto_protocol: fix UnboundLocalError

fix the following crash.
from Yuichi Ito.  minor editorialization by me.

> Traceback (most recent call last):
>   File "/usr/local/bin/ryu-manager", line 9, in <module>
>     load_entry_point('ryu==3.6', 'console_scripts', 'ryu-manager')()
>   File "/usr/local/lib/python2.7/dist-packages/ryu/cmd/manager.py", line 73, in main
>     services.extend(app_mgr.instantiate_apps(**contexts))
>   File "/usr/local/lib/python2.7/dist-packages/ryu/base/app_manager.py", line 368, in instantiate_apps
>     self._instantiate(app_name, cls, *args, **kwargs)                         >   File "/usr/local/lib/python2.7/dist-packages/ryu/base/app_manager.py", line 350, in _instantiate
>     ofproto_protocol.set_app_supported_versions(cls.OFP_VERSIONS)
>   File "/usr/local/lib/python2.7/dist-packages/ryu/ofproto/ofproto_protocol.py", line 40, in set_app_supported_versions
>     _versions &= set(vers)
> UnboundLocalError: local variable '_versions' referenced before assignment