add null check to fix block extraction on x64
add null check to fix block extraction on x64

terry2012/codereason,terry2012/codereason,chubbymaggie/codereason,terry2012/codereason,chubbymaggie/codereason,trailofbits/codereason,trailofbits/codereason,chubbymaggie/codereason,terry2012/codereason,chubbymaggie/codereason,trailofbits/codereason,terry2012/codereason,chubbymaggie/codereason,trailofbits/codereason,trailofbits/codereason