This fixes a bug that our CI system stumbled upon:

+./scripts/create_descriptors 200 --num-resistant-descs 100
Traceback (most recent call last):
  File "./scripts/create_descriptors", line 298, in <module>
    make_descriptors(args.num_descs, args.num_resistant_descs)
  File "./scripts/create_descriptors", line 266, in make_descriptors
    num_probing_resistant > 0)
  File "./scripts/create_descriptors", line 242, in create_extrainfo_desc
    }, signing_key=signing_key)
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/stem/descriptor/extrainfo_descriptor.py", line 944, in create
    return cls(cls.content(attr, exclude, sign, signing_key), validate = validate)
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/stem/descriptor/extrainfo_descriptor.py", line 865, in __init__
    self._parse(entries, validate)
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/stem/descriptor/__init__.py", line 744, in _parse
    parser_for_line[keyword](self, entries)
  File "/home/travis/virtualenv/python2.7.15/lib/python2.7/site-packages/stem/descriptor/extrainfo_descriptor.py", line 296, in _parse_transport_line
    raise ValueError('Transport line has a malformed port: transport %s' % value)
ValueError: Transport line has a malformed port: transport obfs3 203.54.46.87:-2