Crash on triple quoted string with line continuation

[Wilfred]

Given the file:

foo = \
    """bar
"""

pycodestyle 2.3.1 crashes:

$ pycodestyle demo.py
Traceback (most recent call last):
  File "/users/is/whughes/pyenvs/research3/bin/pycodestyle", line 11, in <module>
    sys.exit(_main())
  File "/users/is/whughes/pyenvs/research3/lib/python2.7/site-packages/pycodestyle.py", line 2309, in _main
    report = style_guide.check_files()
  File "/users/is/whughes/pyenvs/research3/lib/python2.7/site-packages/pycodestyle.py", line 2010, in check_files
    runner(path)
  File "/users/is/whughes/pyenvs/research3/lib/python2.7/site-packages/pycodestyle.py", line 2022, in input_file
    return fchecker.check_all(expected=expected, line_offset=line_offset)
  File "/users/is/whughes/pyenvs/research3/lib/python2.7/site-packages/pycodestyle.py", line 1785, in check_all
    self.check_logical()
  File "/users/is/whughes/pyenvs/research3/lib/python2.7/site-packages/pycodestyle.py", line 1663, in check_logical
    for offset, text in self.run_check(check, argument_names) or ():
  File "/users/is/whughes/pyenvs/research3/lib/python2.7/site-packages/pycodestyle.py", line 633, in continued_indentation
    rel_indent[end[0] - first_row] = rel_indent[row]
IndexError: list assignment index out of range

This looks like a regression on #388.