Added missing NULL pointer check

Bug 20676373

Change-Id: Ia99cac8e6abb4157441d8ff1741b5d81981a1270
Reviewed-on: https://swiftshader-review.googlesource.com/3000Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>