Slicecutor: do not pass in last_stmt if it is None.
Slicecutor: do not pass in last_stmt if it is None.

Otherwise no statement will be executed at all due to the way last_stmt
support is implemented in SimEngineVEX.

angr/angr,f-prettyland/angr,axt/angr,iamahuman/angr,axt/angr,schieb/angr,f-prettyland/angr,tyb0807/angr,iamahuman/angr,angr/angr,chubbymaggie/angr,tyb0807/angr,angr/angr,axt/angr,chubbymaggie/angr,tyb0807/angr,chubbymaggie/angr,schieb/angr,f-prettyland/angr,iamahuman/angr,schieb/angr