Fix: init winning_selection before the first jump

When I open new file, every time I do `cmd+;` and then `escape` to cancel (i.e. don't jump anywhere), I have this message in sublime console:
```
Traceback (most recent call last):
  File "./easy_motion.py", line 202, in finish_easy_motion
  File "./easy_motion.py", line 218, in jump_to_winning_selection
AttributeError: 'EasyMotionCommand' object has no attribute 'winning_selection'
```
Initialized `winning_selection` with `None`.