LSL6 crash when dragging empty text area scrollbar

[RayKoopa]

• ScummVM 2.6.0git6340-g974c51b4cd1 (Jun 5 2022 09:42:22)
• Game language: English
• Game version: Hi-Res 1.000.000
• Windows 11 21H2 10.0.22000.675

Steps to reproduce:

• Start a fresh instance of new game (do not "New" / "Load" inside the game).
• Make sure the text area is visible (Game > Text On / Off)
• Attempt to drag the scrollbar thumb downwards.
• The emulator will crash with the following log:

  Assertion failed: idx < _size, file ./common/array.h, line 232
  
  [process exited with code 3 (0x00000003)]
  

In DOSBox, the thumb can be dragged smoothly, despite nothing being scrolled.

Also, the text area is always cleared when loading or saving games. In DOSBox, it retains the last messages, even restoring ones of a save game, despite them being scrolled out of sight (as if an empty message is posted and scrolled to).