Fix assertion if stem can't find a relay

2018-06-06 04:42:03 ERROR Thread-5 scanner.py:225 - dispatch_worker_thread - Unhandled exception in worker thread
Traceback (most recent call last):
  File "/home/pastly/src/simple-bw-scanner/sbws/core/scanner.py", line 223, in dispatch_worker_thread
    return measure_relay(*a, **kw)
  File "/home/pastly/src/simple-bw-scanner/sbws/core/scanner.py", line 138, in measure_relay
    dest = destinations.next()
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/destination.py", line 222, in next
    self._perform_usability_test()
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/destination.py", line 171, in _perform_usability_test
    circ_id = self._cb.build_circuit([None, exit.fingerprint])
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/circuitbuilder.py", line 175, in build_circuit
    for r in path]
  File "/home/pastly/src/simple-bw-scanner/sbws/lib/circuitbuilder.py", line 175, in <listcomp>
    for r in path]
AttributeError: 'NoneType' object has no attribute 'fingerprint'
2018-06-06 04:42:03 ERROR Thread-10 scanner.py:280 - closure - Unhandled exception caught while measuring torbogen: <class 'AttributeError'> 'NoneType' object has no attribute 'fingerprint'