sw test tool: Fix a problem for disconnection
sw test tool: Fix a problem for disconnection

Disconnection of switches has caused a RyuInternalError because target_sw/tester_sw is set to NoneType.
This patch fixes the problem.

Signed-off-by: Yuichi Ito <7a83edf5b53f0b21d4182a3da7518164ce03dc3a@gmail.com>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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