title: Pyshark FileCapture doesn't work
body: <p>I'm new with pyshark, and I write a sample code by searching on the tutorial</p>
  
  <pre><code>import pyshark
  cap = pyshark.FileCapture("input.cap")
  cap_1 = cap[0]
  </code></pre>
  
  <p>and then it give me an error</p>
  
  <pre><code>/Users/tingyugu/anaconda3/bin/python /Users/tingyugu/PycharmProjects/final/test.py
  Traceback (most recent call last):
  File "/Users/tingyugu/anaconda3/lib/python3.6/site-packages/pyshark/capture/file_capture.py", line 70, in __getitem__
  next(self)
  File "/Users/tingyugu/anaconda3/lib/python3.6/site-packages/pyshark/capture/file_capture.py", line 60, in __next__
  packet = self._packet_generator.send(None)
  StopIteration
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
  File "/Users/tingyugu/PycharmProjects/final/test.py", line 5, in &lt;module&gt;
  cap_1 = cap[0]
  File "/Users/tingyugu/anaconda3/lib/python3.6/site-packages/pyshark/capture/file_capture.py", line 73, in __getitem__
  raise KeyError('Packet of index %d does not exist in capture' % packet_index)
  KeyError: 'Packet of index 0 does not exist in capture'
  </code></pre>
  
  <p>I know the reason is that there is no packets in the cap, but my friend can read the file by pyshark</p>
  
  <p>I use the python 3.6.0 anaconda and the pyshark is 0.3.7 in anaconda</p>