TextBoxWidget: Unbreak Wikipedia

[NiLuJe]

Turns out we can't release line_num_to_image early, so,
delay it until the final, full widget teardown.

(Regression since #8250, c.f.,
#8250 (comment))

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This change is 

[poire-z]

In this week's Turns out we can't.... episode:

./luajit: frontend/ui/widget/textboxwidget.lua:771: attempt to get length of field 'vertical_string_list' (a nil value)
stack traceback:
        frontend/ui/widget/textboxwidget.lua:771: in function '_renderText'
        frontend/ui/widget/textboxwidget.lua:867: in function '_updateLayout'
        frontend/ui/widget/textboxwidget.lua:1156: in function 'update'
        frontend/ui/widget/textboxwidget.lua:998: in function 'action'

I can't find out 100% reproducible steps. But twice I got it via Wikipedia history > hit past lookup "foo" > browse 2 results with images, Wikipedia full on the 2nd ("Dave Grohl"), close the full result, and crash.

So, @ N̷i̼̪̪̖͖ḽ̣̠̺̜'̪̯̘̙̱͞u̧͖̭̜͚̪j͙̝̩̝̪̖e͉͝ T̻͓̺͎̱͙̫h̹͇e̛ ̗̭N̤͉͎͉̬il̠̱͡'̼i̢̙̟̝̹͉f҉̙̝̝͉icà̳̰̠̬͚t̹̼̻͟ò͓͈͓̥r҉̫̘͍ ͚̹̞ͅ ! I implore you in fear and trembling: š͚̪̐ṯ̻̥̹̓ͮ̿ͤ̄o̖ͧͤͭ̑̒̽̓ͅp̦̱͎̳͍̣̞ͩ̄͐̾ ͍̙̺̗̆̿̅ͧͯ̑̓ṇ̗i̳̖̰l̗ͅ'̳̭͔͕̤̤̉͊͌̂ĩ̱͔̠̫̒̀f̤̗̮̖̤͈̒ͅý̪̖̎͆̉̚í̼n̥̮̪̑ͧ̎͋̽ͅg̙̜͕͍͚͒̅̔̍ ̬͇ͨ̍ͧ̒ͮ̇e̪̹̼̩̦ͩ̍v̖̘̑̔́͊ͯͮ̚e̪͖͙̝͐͐ͥ͆̊r̦̲̖̘̰̦̊y͈͔̅̃ͤͮͪ̅̔t̞̝̃̃̒ͤ̉ͤh̫͈̝͎̬̙͐̎̇̌̑̿i̓n͉̼̟̟̫̰̉ͫ̊ͬ͋ͪͩgͧ̈́ !

[NiLuJe]

That... looks like an actual bug (i.e., a widget reused after a full :free(true) and not re-init'ed?).

[NiLuJe]

Oh, wait, no. _splitToLines uses a blank table to reset, not a nil ;).

[poire-z]

An empty table instead of a nil, you're getting soft :)
(Still can't manage to reproduce it, so can't check if this would have solved it.)