vfg: check if ignored_variable is None
vfg: check if ignored_variable is None

simuvex/plugins/scratch.py set ignored_variables to None, we need to
check it otherwise we might trying to call copy() on non-initialized
object.

chubbymaggie/angr,cureHsu/angr,cureHsu/angr,iamahuman/angr,fjferrer/angr,zhuyue1314/angr,angr/angr,chubbymaggie/angr,schieb/angr,tyb0807/angr,xurantju/angr,chubbymaggie/angr,mingderwang/angr,xurantju/angr,tyb0807/angr,avain/angr,lowks/angr,lowks/angr,terry2012/angr,GuardianRG/angr,iamahuman/angr,angr/angr,f-prettyland/angr,fjferrer/angr,tyb0807/angr,f-prettyland/angr,terry2012/angr,iamahuman/angr,avain/angr,schieb/angr,angr/angr,axt/angr,haylesr/angr,axt/angr,mingderwang/angr,GuardianRG/angr,schieb/angr,f-prettyland/angr,zhuyue1314/angr,axt/angr,haylesr/angr