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

It doesn't look right if stream with wrong state is passed into the core DAWG library. It causes situation:
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
    ...