Fix some crashes in Snapshot view

[gnufied]

I mainly saw two crashes:

--> 
    __TEXT                 000000010799d000-0000000107a59000 [  752K] r-x/rwx SM=COW  /Users/USER/*/RbkitClient.app/Contents/MacOS/RbkitClient

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   ???                             000000000000000000 0 + 0
1   org.qt-project.QtCore           0x000000010889745a QAbstractItemModel::hasIndex(int, int, QModelIndex const&) const + 42
2   codemancers.RbkitClient         0x00000001079b452e RBKit::HeapDataModel::index(int, int, QModelIndex const&) const + 46
3   org.qt-project.QtCore           0x00000001088b3da7 QIdentityProxyModel::qt_metacall(QMetaObject::Call, int, void**) + 10231
4   org.qt-project.QtCore           0x00000001088bdb22 QSortFilterProxyModel::mapToSource(QModelIndex const&) const + 18
5   org.qt-project.QtCore           0x00000001088bc7fa QSortFilterProxyModel::flags(QModelIndex const&) const + 90
6   org.qt-project.QtWidgets        0x0000000107de03f3 QTreeView::drawRow(QPainter*, QStyleOptionViewItem const&, QModelIndex const&) const + 2611
7   org.qt-project.QtWidgets        0x0000000107dde6b7 QTreeView::drawTree(QPainter*, QRegion const&) const + 1271
8   org.qt-project.QtWidgets        0x0000000107dde0fd QTreeView::paintEvent(QPaintEvent*) + 349
9   org.qt-project.QtWidgets        0x0000000107b8c4bb QWidget::event(QEvent*) + 2123
10  org.qt-project.QtWidgets        0x0000000107c78edd QFrame::event(QEvent*) + 45
11  org.qt-project.QtWidgets        0x0000000107cfc1fa QAbstractScrollArea::viewportEvent(QEvent*) + 170
12  org.qt-project.QtWidgets        0x0000000107d9d11d QAbstractItemView::viewportEvent(QEvent*) + 1453
13  org.qt-project.QtWidgets        0x0000000107ddde71 QTreeView::viewportEvent(QEvent*) + 513
14  org.qt-project.QtWidgets        0x0000000107cfccf5 0x107b1b000 + 1973493
15  org.qt-project.QtCore           0x00000001088e62f9 QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject*, QEvent*) + 217
16  org.qt-project.QtWidgets        0x0000000107b4cdab QApplicationPrivate::notify_helper(QObject*, QEvent*) + 235
17  org.qt-project.QtWidgets        0x0000000107b50110 QApplication::notify(QObject*, QEvent*) + 8192
18  org.qt-project.QtCore           0x00000001088e5fb3 QCoreApplication::notifyInternal(QObject*, QEvent*) + 115

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   codemancers.RbkitClient         0x000000010553a6dc RBKit::HeapDataModel::rowCount(QModelIndex const&) const + 44
1   org.qt-project.QtCore           0x000000010644662f QSortFilterProxyModel::columnCount(QModelIndex const&) const + 95
2   org.qt-project.QtWidgets        0x000000010593a037 QHeaderView::initializeSections() + 103
3   org.qt-project.QtWidgets        0x0000000105928ad2 QAbstractItemView::setModel(QAbstractItemModel*) + 1282
4   org.qt-project.QtWidgets        0x0000000105939fac QHeaderView::setModel(QAbstractItemModel*) + 956
5   org.qt-project.QtWidgets        0x0000000105967746 QTreeView::setModel(QAbstractItemModel*) + 374
6   codemancers.RbkitClient         0x0000000105543034 HeapDumpForm::loadFromSpecifiedRoot(RBKit::BaseHeapItem*) + 116
7   codemancers.RbkitClient         0x000000010554346e HeapDumpForm::viewParents() + 78
8   org.qt-project.QtCore           0x00000001064a39db QMetaObject::activate(QObject*, int, int, void**) + 2987
9   org.qt-project.QtWidgets        0x00000001056d1eff QAction::activate(QAction::ActionEvent) + 271
10  org.qt-project.QtWidgets        0x00000001058431c2 0x1056a9000 + 1679810
11  org.qt-project.QtWidgets        0x0000000105841742 0x1056a9000 + 1673026
12  org.qt-project.QtWidgets        0x000000010571a2cc QWidget::event(QEvent*) + 1628
13  org.qt-project.QtWidgets        0x00000001058476e6 QMenu::event(QEvent*) + 950
``

I think they are mainly because of invalid items being selected from the view and we are not checking for it.

[iffyuva]

unable to reproduce this. but will keep an eye.