Fix UnboundLocalError in Log class

[keeferrourke]

I came across the following error while building a dashing dashboard (paths truncated in traceback for brevity):

  File "venv/lib/python3.9/site-packages/dashing/dashing.py", line 100, in display
    self._display(tbox, None)
  File "venv/lib/python3.9/site-packages/dashing/dashing.py", line 144, in _display
    i._display(TBox(tbox.t, x, y, item_width, item_height), self)
  File "venv/lib/python3.9/site-packages/dashing/dashing.py", line 144, in _display
    i._display(TBox(tbox.t, x, y, item_width, item_height), self)
  File "venv/lib/python3.9/site-packages/dashing/dashing.py", line 144, in _display
    i._display(TBox(tbox.t, x, y, item_width, item_height), self)
  File "venv/lib/python3.9/site-packages/dashing/dashing.py", line 205, in _display
    if i < tbox.h:
UnboundLocalError: local variable 'i' referenced before assignment

This patch is a quick fix for this particular error.