Potential null pointer dereference in MFStream

There is a bug in MFStream::Seek regarding the parameter pqwCurrentPosition.
Instead of checking against NULL, the function dereferences the pointer:

    if (*pqwCurrentPosition)
        *pqwCurrentPosition = pos;

https://github.com/qt/qtmultimedia/blob/dev/src/plugins/wmf/mfstream.cpp#L239

This will crash if NULL is passed, or fail to return the new position if the given pointer points to a zero-initialized value.