Fix for checking key causes SEGFAULT after trying to load non-existing file

***important  notice***: these changes don't contains updated CPP files and build fails without them, but i don't commit CPP files because want to merge only source changes and i don't know about cython policies for this project (i have 0.19.1, but i saw old CPP files has been generated by 0.18)
It doesn't look right if stream with wrong state is passed into the core DAWG library. It causes situation:
```python
d = IntDAWG()
try:
  d.load('some_wrong_path') # load optional tree
except:
   ignore errors, make logging or another stuff

if 'any_key' in d: # it will be core dumped here
    ...
```