Fix incorrect assertion and possible nullref

[smoogipoo]

I've had the following osu! side tests fail when using a local framework checkout:

TestConfineAlwaysInFullscreen
TestConfineAlwaysUserSetting

 at osu.Framework.Logging.ThrowingTraceListener.Fail(String message1, String message2) in /Users/smgi/Repos/osu-framework/osu.Framework/Logging/ThrowingTraceListener.cs:line 27
   at System.Diagnostics.TraceInternal.Fail(String message, String detailMessage)
   at System.Diagnostics.Debug.Fail(String message, String detailMessage)
   at osu.Framework.Graphics.UserInterface.TextBox.unbindInput(TextBox next) in /Users/smgi/Repos/osu-framework/osu.Framework/Graphics/UserInterface/TextBox.cs:line 1380
   at osu.Framework.Graphics.UserInterface.TextBox.Dispose(Boolean isDisposing) in /Users/smgi/Repos/osu-framework/osu.Framework/Graphics/UserInterface/TextBox.cs:line 539
   at osu.Framework.Graphics.Drawable.Dispose() in /Users/smgi/Repos/osu-framework/osu.Framework/Graphics/Drawable.cs:line 80

Basically, this can happen if the TextBox is disposed before getting loaded.