bug#12140: 24.1; mouse-drag-track: Args out of range: 2746, 71308

[Leo]

1. Emacs -q
2. C-h i m elisp RET
3. Use mouse-1 to click the buffer
3. 1 ; enter the first node
4. Use mouse-1 to click the buffer

Debugger entered--Lisp error: (args-out-of-range 1 71543)
  buffer-substring-no-properties(1 71543)
  mouse-drag-track((down-mouse-1 (#<window 3 on *info*> 71580 (291 . 474) 
214100214 nil 71580 (36 . 31) nil (3 . 14) (8 . 15))) t)
  mouse-drag-region((down-mouse-1 (#<window 3 on *info*> 71580 (291 . 474) 
214100214 nil 71580 (36 . 31) nil (3 . 14) (8 . 15))))
  call-interactively(mouse-drag-region nil nil)