Some QFileSystemMetaData members are not initialized when using iterators

In QFileEngineIterator::advance implementations, the accessTime_ member of the QFileSystemMetaData out-parameter might remain uninitialized.

Either the QFileSystemMetaData default constructor should initialize all members, or the platform specific implementation of advance should zero out members that are not populated.

See Coverity issues 11399 and 11419.

A previous attempt to address the issue has been abandoned:

• https://codereview.qt-project.org/c/qt/qtbase/+/112336

Given the comments, and the proposed solutions, zeroing out the accessTime_ member in the Windows implementation of QFileSystemIterator::advance might be sufficient.