fix a crash

Dependencies

• [2] BOFNXP5G clicking now moves the cursor even on long, wrapped lines
• [3] 5L7K4GBD clicking to the right of a wrapped line
• [4] 5FW7YOFT highlight selection while dragging
• [*] BULPIBEG beginnings of a module for the text editor

Change contents

• replacement in text.lua at line 677
  [2.628]→[2.628:653](∅→∅)
  − assert(line.fragments)
  [2.628]

  [2.653]
  + if line.fragments == nil then

  + Text.compute_fragments(line, Line_width)

  + end