Spell-checker crashes if a regex('^ +#') line in a docstring contains a typo

[gschaffner]

Steps to reproduce

1. pip install pyenchant
2. Create the file test.py with contents

   def foo():
       """
       # tpyo
       """

3. pylint --disable all --enable spelling --spelling-dict en_US test.py

Current behavior

Traceback (most recent call last):
  File "/home/ganden/.pyenv/versions/jlab-test/bin/pylint", line 10, in <module>
    sys.exit(run_pylint())
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/__init__.py", line 22, in run_pylint
    PylintRun(sys.argv[1:])
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/lint/run.py", line 349, in __init__
    linter.check(args)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 862, in check
    self._check_files(
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 896, in _check_files
    self._check_file(get_ast, check_astroid_module, name, filepath, modname)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 922, in _check_file
    check_astroid_module(ast_node)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 1054, in check_astroid_module
    retval = self._check_astroid_module(
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/lint/pylinter.py", line 1099, in _check_astroid_module
    walker.walk(ast_node)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/utils/ast_walker.py", line 75, in walk
    self.walk(child)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/utils/ast_walker.py", line 72, in walk
    callback(astroid)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/checkers/spelling.py", line 397, in visit_functiondef
    self._check_docstring(node)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/checkers/spelling.py", line 411, in _check_docstring
    self._check_spelling("wrong-spelling-in-docstring", line, start_line + idx)
  File "/home/ganden/.pyenv/versions/3.8.3/envs/jlab-test/lib/python3.8/site-packages/pylint/checkers/spelling.py", line 347, in _check_spelling
    col = line_segment.index(word)
ValueError: substring not found

Expected behavior

Find tpyo correctly.

pylint --version output

pylint 2.6.0
astroid 2.4.2
Python 3.8.3 (default, Jul 24 2020, 19:51:30)
[GCC 10.1.0]
# and also
pyenchant 3.1.1
enchant 2.2.8
nuspell 3.1.2

More info

The following do not crash pylint (they behave as expected):

def foo():
    """# tpyo"""

"""# tpyo"""

def foo():
    """
# tpyo
    """

def foo():
    """
    # cat
    """

def foo():
    """
    tpyo # cat
    """

def foo():
    """
    cat # tpyo
    """