of: Prevent __str__() methods from raising exceptions
of: Prevent __str__() methods from raising exceptions

ofproto_parser.MsgBase.__str__(): avoid "TypeError: %x format: a number is required, not NoneType"

zyq001/ryu,takahashiminoru/ryu,haniehrajabi/ryu,StephenKing/summerschool-2015-ryu,fkakuma/ryu,gopchandani/ryu,StephenKing/ryu,Tejas-Subramanya/RYU_MEC,darjus-amzn/ryu,zyq001/ryu,fkakuma/ryu,OpenState-SDN/ryu,gareging/SDN_Framework,openvapour/ryu,Tesi-Luca-Davide/ryu,lagopus/ryu-lagopus-ext,ysywh/ryu,StephenKing/ryu,openvapour/ryu,elahejalalpour/ELRyu,fujita/ryu,Tesi-Luca-Davide/ryu,lsqtongxin/ryu,OpenState-SDN/ryu,jalilm/ryu,zangree/ryu,takahashiminoru/ryu,StephenKing/summerschool-2015-ryu,diogommartins/ryu,fujita/ryu,ysywh/ryu,StephenKing/summerschool-2015-ryu,haniehrajabi/ryu,osrg/ryu,habibiefaried/ryu,iwaseyusuke/ryu,habibiefaried/ryu,fujita/ryu,darjus-amzn/ryu,ynkjm/ryu,zyq001/ryu,OpenState-SDN/ryu,lsqtongxin/ryu,diogommartins/ryu,gareging/SDN_Framework,ysywh/ryu,sivaramakrishnansr/ryu,openvapour/ryu,alanquillin/ryu,iwaseyusuke/ryu,sivaramakrishnansr/ryu,shinpeimuraoka/ryu,StephenKing/ryu,gareging/SDN_Framework,John-Lin/ryu,TakeshiTseng/ryu,zangree/ryu,yamt/ryu,diogommartins/ryu,shinpeimuraoka/ryu,ynkjm/ryu,alanquillin/ryu,lagopus/ryu-lagopus-ext,fkakuma/ryu,habibiefaried/ryu,diogommartins/ryu,StephenKing/ryu,ttsubo/ryu,John-Lin/ryu,umkcdcrg01/ryu_openflow,fujita/ryu,diogommartins/ryu,pichuang/ryu,yamt/ryu,TakeshiTseng/ryu,osrg/ryu,hisaharu/ryu,sivaramakrishnansr/ryu,Tejas-Subramanya/RYU_MEC,osrg/ryu,gopchandani/ryu,hisaharu/ryu,pichuang/ryu,Zouyiran/ryu,lsqtongxin/ryu,elahejalalpour/ELRyu,lagopus/ryu-lagopus-ext,habibiefaried/ryu,zyq001/ryu,takahashiminoru/ryu,iwaseyusuke/ryu,Tejas-Subramanya/RYU_MEC,TakeshiTseng/ryu,yamt/ryu,gopchandani/ryu,jalilm/ryu,Tejas-Subramanya/RYU_MEC,StephenKing/summerschool-2015-ryu,zangree/ryu,ysywh/ryu,hisaharu/ryu,hisaharu/ryu,Tesi-Luca-Davide/ryu,ynkjm/ryu,alanquillin/ryu,ttsubo/ryu,darjus-amzn/ryu,iwaseyusuke/ryu,lsqtongxin/ryu,OpenState-SDN/ryu,alanquillin/ryu,gareging/SDN_Framework,elahejalalpour/ELRyu,haniehrajabi/ryu,shinpeimuraoka/ryu,umkcdcrg01/ryu_openflow,lagopus/ryu-lagopus-ext,StephenKing/ryu,John-Lin/ryu,osrg/ryu,openvapour/ryu,umkcdcrg01/ryu_openflow,lsqtongxin/ryu,darjus-amzn/ryu,Tejas-Subramanya/RYU_MEC,ynkjm/ryu,ttsubo/ryu,jalilm/ryu,sivaramakrishnansr/ryu,John-Lin/ryu,Zouyiran/ryu,shinpeimuraoka/ryu,umkcdcrg01/ryu_openflow,Zouyiran/ryu,jalilm/ryu,TakeshiTseng/ryu,pichuang/ryu,Tesi-Luca-Davide/ryu,Zouyiran/ryu,gareging/SDN_Framework,ttsubo/ryu,habibiefaried/ryu,Tesi-Luca-Davide/ryu,haniehrajabi/ryu,iwaseyusuke/ryu,pichuang/ryu,elahejalalpour/ELRyu,StephenKing/summerschool-2015-ryu,zangree/ryu,ynkjm/ryu,elahejalalpour/ELRyu,osrg/ryu,jalilm/ryu,yamt/ryu,haniehrajabi/ryu,pichuang/ryu,OpenState-SDN/ryu,ttsubo/ryu,umkcdcrg01/ryu_openflow,ysywh/ryu,Zouyiran/ryu,lagopus/ryu-lagopus-ext,yamt/ryu,zyq001/ryu,darjus-amzn/ryu,John-Lin/ryu,openvapour/ryu,gopchandani/ryu,alanquillin/ryu,hisaharu/ryu,fkakuma/ryu,shinpeimuraoka/ryu,fujita/ryu,takahashiminoru/ryu,gopchandani/ryu,fkakuma/ryu,takahashiminoru/ryu,sivaramakrishnansr/ryu,TakeshiTseng/ryu,zangree/ryu