data handler receiving incomplete data should be handled gracefully

[vegu]

Split out from 20c/vaping#12

Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/gevent/greenlet.py", line 536, in run
    result = self._run(*self.args, **self.kwargs)
  File "/usr/lib/python2.7/site-packages/vaping/plugins/__init__.py", line 134, in _run
    self._emit.emit(msg)
  File "/usr/lib/python2.7/site-packages/vaping/plugins/vodka.py", line 56, in emit
    vodka.data.handle(data.get("type"), data, data_id=data.get("source"), caller=self)
  File "/usr/lib/python2.7/site-packages/vodka/data/__init__.py", line 34, in handle
    data = handler(data, caller=caller)
  File "/usr/lib/python2.7/site-packages/vodka/data/handlers.py", line 93, in __call__
    for d in data["data"]])
TypeError: 'NoneType' object has no attribute '__getitem__'
Wed Apr 19 12:56:16 2017 <FPing at 0x7f1f27667d70> failed with TypeError