some temporary logging to catch a bug

some temporary logging to catch a bug

The bug has been spotted twice:

1. In snap.love, I selected text in one node, then another, and hit:
  Error: text.lua:789: attempt to compare nil with number
  stack traceback:
    text.lua:789: in function 'lt1'
    select.lua:19: in function 'clip_selection'
    text.lua:32: in function 'draw'
    edit.lua:117: in function 'draw'
    [string "REPL"]:21: in function 'draw'
    main.lua:152: in function 'draw'
    app.lua:102: in function <app.lua:84>
    [C]: in function 'xpcall'
    app.lua:112: in function <app.lua:111>
    [C]: in function 'xpcall'

  Couldn't reproduce.

2. In text.love, inscript selected all text in a small buffer and then
   clicked outside the text. And got:

  Error: text.lua:784: attempt to compare nil with number
  Traceback
    [love "callbacks.lua"]:228: in function 'handler'
    text.lua:784: in function 'lt1'
    select.lua:19: in function 'clip_selection'
    text.lua:27: in function 'draw'
    edit.lua:117: in function 'draw'
    run.lua:136: in function 'draw'
    main.lua:148: in function 'draw'
    app.lua:42: in function <app.lua:22>
    [C]: in function 'xpcall'

  This is reproducible, and also across forks.