Fix mismatched new[] / delete[] valgrind warning

QScopedPointer by default uses `delete`, even if we used `new []`.
We would have to use the following verbose syntax:

    QScopedPointer<uchar, QScopedPointerArrayDeleter<uchar>> buf(new uchar[...]);

But `std::unique_ptr` can do the right thing by just looking at the template
parameter, so let's use that.