Avoid NULL pointer exceptions when blitting without a buffer

Change-Id: Ic670895f50ff3aa12fb0963e321541b21e886e12
Reviewed-on: https://swiftshader-review.googlesource.com/3110
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
1 file changed