GridComicsView: ignore unhandled key presses

When a key not handled by Grid view is pressed while it has focus, the
following warning appears in Library's output
qrc:/qml/GridComicsView.qml:776: Error: Cannot assign [undefined] to int
AND the top-left cover is selected. The added early return fixes both
issues.

YACReaderLibrary/qml/GridComicsView.qml

@@ -806,6 +806,8 @@ Rectangle {
             }
             else if (event.key === Qt.Key_Down) {
                 ci = Math.min(grid.currentIndex+numCells,grid.count - 1);
+            } else {
+                return;
             }
 
             event.accepted = true;