wxGUI/vdigit: fix double click on the line/boundary after activate 'Move selected vertex' editing tool and then switch to 'Edit selected line/boundary' editing tool

[tmszi]

To Reproduce
Steps to reproduce the behavior:

1. Launch wxGUI Vector Digitizer: g.gui.vdigit -c test
2. Draw some line
3. Activate 'Move selected vertex' editing tool
4. Double left click on the line
5. Activate (switch to) 'Edit selected line/boundary' editing tool
6. You get error message

Error message

Traceback (most recent call last):
  File "/usr/lib64/grass79/gui/wxpython/mapwin/buffered.py", line 1430, in MouseActions
    self.OnMouseMoving(event)
  File "/usr/lib64/grass79/gui/wxpython/mapwin/buffered.py", line 1703, in OnMouseMoving
    self._onMouseMoving(event)
  File "/usr/lib64/grass79/gui/wxpython/vdigit/mapwindow.py", line 1246, in _onMouseMoving
    pdc=self.pdcTmp, begin=self.Cell2Pixel(self.polycoords[-1])
IndexError: list index out of range

Expected behavior

No error message after double-clicking on the line/boundary, after activate the 'Move selected vertex' editing tool and then activate (switch to) 'Edit selected line/boundary' editing tool.