Piano: Avoid selecting out of range notes.

Fixes #5736. The selected note value could also underflow if
you drag to the left, but the assert got triggered only in
case you're dragging past the end of the note roll.

Userland/Applications/Piano/RollWidget.cpp

@@ -191,7 +191,7 @@ void RollWidget::mousemove_event(GUI::MouseEvent& event)
         if (note_width_is_fractional && x_is_not_last)
             ++x;
         x /= m_note_width;
-        return x;
+        return clamp(x, 0, m_num_notes - 1);
     };
 
     int x0 = get_note_x(m_note_drag_start.value().x());