fix crash when using up arrow

#0 raise() at /usr/lib/libc.so.6
#1 abort() at /usr/lib/libc.so.6
#2 __assert_fail_base.cold() at /usr/lib/libc.so.6
#3 __assert_fail() at /usr/lib/libc.so.6
#4 DBDataModel::index() at /home/michal/projects/sources/broom/src/gui/desktop/models/db_data_model.cpp:313
#5 QAbstractItemModel::sibling(int, int, QModelIndex const&) const() at /usr/lib/libQt5Core.so.5
#6 QModelIndex::sibling() at /usr/include/qt/QtCore/qabstractitemmodel.h:444
#7 Data::getTopOf() at /home/michal/projects/sources/broom/src/gui/desktop/views/view_impl/data.cpp:359
#8 ImagesTreeView::moveCursor() at /home/michal/projects/sources/broom/src/gui/desktop/views/images_tree_view.cpp:177

Data::getTopOf() was asking for index with negative row (which
is +- ok). DBDataModel::index() was not ready for it.

src/gui/desktop/models/db_data_model.cpp

@@ -310,20 +310,23 @@ QVariant DBDataModel::data(const QModelIndex& _index, int role) const
 
 QModelIndex DBDataModel::index(int row, int column, const QModelIndex& _parent) const
 {
-    assert(row >= 0);
-    const unsigned int urow = static_cast<unsigned int>(row);
-
     QModelIndex idx;
-    IIdxData* pDataRaw = m_idxDataManager->getIdxDataFor(_parent);
-
-    assert(::isNode(pDataRaw));
-    IdxNodeData* pData = static_cast<IdxNodeData *>(pDataRaw);
 
-    if (urow < pData->getChildren().size())             //row out of boundary?
+    if (column >= 0 && row >= 0)
     {
-        const std::vector<IIdxData::Ptr>& children = pData->getChildren();
-        IIdxData* cData = children[urow].get();
-        idx = createIndex(row, column, cData);
+        const unsigned int urow = static_cast<unsigned int>(row);
+
+        IIdxData* pDataRaw = m_idxDataManager->getIdxDataFor(_parent);
+
+        assert(::isNode(pDataRaw));
+        IdxNodeData* pData = static_cast<IdxNodeData *>(pDataRaw);
+
+        if (urow < pData->getChildren().size())             //row out of boundary?
+        {
+            const std::vector<IIdxData::Ptr>& children = pData->getChildren();
+            IIdxData* cData = children[urow].get();
+            idx = createIndex(row, column, cData);
+        }
     }
 
     return idx;