Make the correct row visible when selecting rows with shift + arrow keys in the grid. Closes #1533.

aegisub/src/base_grid.cpp

@@ -1045,7 +1045,7 @@ void BaseGrid::OnKeyDown(wxKeyEvent &event) {
 
 		SetSelectedSet(newsel);
 
-		MakeCellVisible(extendRow, 0, false);
+		MakeCellVisible(next, 0, false);
 		return;
 	}
 }