eliminate some warnings of jumping on uninitialized values

if not building in the DEBUG mode, the uninitialised values are not guaranteed to
be zero-valued

valgrind logs available:
```
==22355== Conditional jump or move depends on uninitialised value(s)
==22355==    at 0x4CBED6: SeafileTrayIcon::setState(SeafileTrayIcon::TrayState,
QString const&) (tray-icon.cpp:234)
==22355==    by 0x4CA742: SeafileTrayIcon::SeafileTrayIcon(QObject*)
(tray-icon.cpp:69)
==22355==    by 0x483951: SeafileApplet::SeafileApplet()
(seafile-applet.cpp:155)
==22355==    by 0x482DA9: main (main.cpp:143)
==22355==
==22355== Conditional jump or move depends on uninitialised value(s)
==22355==    at 0x4D5AFC: LocalRepo::operator==(LocalRepo const&) const
(local-repo.h:47)
==22355==    by 0x4D5B92: LocalRepo::operator!=(LocalRepo const&) const
(local-repo.h:55)
==22355==    by 0x4D4BAA: RepoTreeModel::refreshRepoItem(RepoItem*, void*)
(repo-tree-model.cpp:315)
==22355==    by 0x4D497C: RepoTreeModel::forEachRepoItem(void
(RepoTreeModel::*)(RepoItem*, void*), void*) (repo-tree-model.cpp:284)
```