lifter: fix the issue that only specifying 'num_inst' raises an exception in PyVEX
lifter: fix the issue that only specifying 'num_inst' raises an exception in PyVEX

cureHsu/angr,fjferrer/angr,axt/angr,angr/angr,tyb0807/angr,angr/angr,f-prettyland/angr,iamahuman/angr,tyb0807/angr,schieb/angr,chubbymaggie/angr,GuardianRG/angr,haylesr/angr,f-prettyland/angr,haylesr/angr,axt/angr,chubbymaggie/angr,fjferrer/angr,iamahuman/angr,GuardianRG/angr,cureHsu/angr,schieb/angr,axt/angr,chubbymaggie/angr,angr/angr,tyb0807/angr,iamahuman/angr,f-prettyland/angr,schieb/angr