Fix zero width textbox throwing in updateImeWindowPosition()

[Susko3]

Math.Clamp could fail if the usable text area was considered negative ($\textup{DrawWidth} - 2 \times \textup{LeftRightPadding} < 0$).

Spotted on discord:

System.ArgumentException: '16' cannot be greater than -16.
   at System.Math.ThrowMinMaxException[T](T min, T max)
   at osu.Framework.Graphics.UserInterface.TextBox.updateImeWindowPosition()
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()