ofproto_protocol: fix UnboundLocalError
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

Signed-off-by: YAMAMOTO Takashi <458bba2e4a8704776b33478100c98529b639cd9e@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

shinpeimuraoka/ryu,Tejas-Subramanya/RYU_MEC,jazzmes/ryu,torufuru/OFPatchPanel,o3project/ryu-oe,John-Lin/ryu,sivaramakrishnansr/ryu,iwaseyusuke/ryu,shinpeimuraoka/ryu,alanquillin/ryu,evanscottgray/ryu,lsqtongxin/ryu,o3project/ryu-oe,darjus-amzn/ryu,habibiefaried/ryu,OpenState-SDN/ryu,Zouyiran/ryu,John-Lin/ryu,muzixing/ryu,zyq001/ryu,sivaramakrishnansr/ryu,lzppp/mylearning,ysywh/ryu,gopchandani/ryu,lzppp/mylearning,ynkjm/ryu,Zouyiran/ryu,ysywh/ryu,jalilm/ryu,StephenKing/summerschool-2015-ryu,shinpeimuraoka/ryu,lsqtongxin/ryu,gopchandani/ryu,iwaseyusuke/ryu,openvapour/ryu,StephenKing/ryu,takahashiminoru/ryu,ynkjm/ryu,jalilm/ryu,lzppp/mylearning,yamt/ryu,gopchandani/ryu,Zouyiran/ryu,StephenKing/summerschool-2015-ryu,zangree/ryu,habibiefaried/ryu,lagopus/ryu-lagopus-ext,John-Lin/ryu,osrg/ryu,lsqtongxin/ryu,gopchandani/ryu,jalilm/ryu,castroflavio/ryu,lzppp/mylearning,muzixing/ryu,ysywh/ryu,ynkjm/ryu,TakeshiTseng/ryu,habibiefaried/ryu,zyq001/ryu,StephenKing/ryu,Tesi-Luca-Davide/ryu,torufuru/oolhackathon,alyosha1879/ryu,openvapour/ryu,darjus-amzn/ryu,TakeshiTseng/ryu,ttsubo/ryu,jazzmes/ryu,habibiefaried/ryu,gareging/SDN_Framework,haniehrajabi/ryu,umkcdcrg01/ryu_openflow,yamt/ryu,yamada-h/ryu,fkakuma/ryu,zangree/ryu,torufuru/OFPatchPanel,OpenState-SDN/ryu,castroflavio/ryu,TakeshiTseng/ryu,jkoelker/ryu,fkakuma/ryu,haniehrajabi/ryu,alyosha1879/ryu,alanquillin/ryu,umkcdcrg01/ryu_openflow,StephenKing/ryu,yamt/ryu,pichuang/ryu,jkoelker/ryu,sivaramakrishnansr/ryu,jazzmes/ryu,pichuang/ryu,ynkjm/ryu,ntts-clo/ryu,elahejalalpour/ELRyu,takahashiminoru/ryu,alanquillin/ryu,shinpeimuraoka/ryu,Zouyiran/ryu,diogommartins/ryu,alanquillin/ryu,pichuang/ryu,fujita/ryu,haniehrajabi/ryu,zangree/ryu,evanscottgray/ryu,haniehrajabi/ryu,John-Lin/ryu,ntts-clo/ryu,iwaseyusuke/ryu,alyosha1879/ryu,lagopus/ryu-lagopus-ext,gareging/SDN_Framework,ynkjm/ryu,lagopus/ryu-lagopus-ext,Tejas-Subramanya/RYU_MEC,ttsubo/ryu,sivaramakrishnansr/ryu,pichuang/ryu,fkakuma/ryu,diogommartins/ryu,hisaharu/ryu,sivaramakrishnansr/ryu,jalilm/ryu,Tesi-Luca-Davide/ryu,Tejas-Subramanya/RYU_MEC,StephenKing/ryu,TakeshiTseng/ryu,OpenState-SDN/ryu,ttsubo/ryu,elahejalalpour/ELRyu,alanquillin/ryu,habibiefaried/ryu,umkcdcrg01/ryu_openflow,zyq001/ryu,gareging/SDN_Framework,TakeshiTseng/ryu,ntts-clo/mld-ryu,lagopus/ryu-lagopus-ext,diogommartins/ryu,torufuru/oolhackathon,torufuru/oolhackathon,OpenState-SDN/ryu,fujita/ryu,ysywh/ryu,openvapour/ryu,fujita/ryu,fujita/ryu,iwaseyusuke/ryu,ysywh/ryu,diogommartins/ryu,elahejalalpour/ELRyu,takahashiminoru/ryu,mikhaelharswanto/ryu,lagopus/ryu-lagopus-ext,zyq001/ryu,iwaseyusuke/ryu,muzixing/ryu,castroflavio/ryu,ttsubo/ryu,pichuang/ryu,hisaharu/ryu,gopchandani/ryu,hisaharu/ryu,evanscottgray/ryu,shinpeimuraoka/ryu,haniehrajabi/ryu,ttsubo/ryu,muzixing/ryu,osrg/ryu,yamt/ryu,lsqtongxin/ryu,openvapour/ryu,Tejas-Subramanya/RYU_MEC,lsqtongxin/ryu,darjus-amzn/ryu,gareging/SDN_Framework,StephenKing/summerschool-2015-ryu,jkoelker/ryu,alyosha1879/ryu,Zouyiran/ryu,elahejalalpour/ELRyu,darjus-amzn/ryu,StephenKing/summerschool-2015-ryu,umkcdcrg01/ryu_openflow,takahashiminoru/ryu,muzixing/ryu,OpenState-SDN/ryu,hisaharu/ryu,StephenKing/ryu,John-Lin/ryu,darjus-amzn/ryu,yamt/ryu,elahejalalpour/ELRyu,Tesi-Luca-Davide/ryu,Tesi-Luca-Davide/ryu,mikhaelharswanto/ryu,umkcdcrg01/ryu_openflow,osrg/ryu,zangree/ryu,takahashiminoru/ryu,yamada-h/ryu,fkakuma/ryu,zangree/ryu,Tesi-Luca-Davide/ryu,StephenKing/summerschool-2015-ryu,osrg/ryu,diogommartins/ryu,lzppp/mylearning,hisaharu/ryu,Tejas-Subramanya/RYU_MEC,gareging/SDN_Framework,fkakuma/ryu,fujita/ryu,jalilm/ryu,openvapour/ryu,ntts-clo/mld-ryu,zyq001/ryu,osrg/ryu