Fix normalizer crash on comments after line continuations

AUTHORS.txt

@@ -56,4 +56,6 @@ Batuhan Taskaya (@isidentical) <isidentical@gmail.com>
 Jocelyn Boullier (@Kazy) <jocelyn@boullier.bzh>
 
 
+salpenr (@salapenar-max)
+
 Note: (@user) means a github user name.

parso/python/pep8.py

@@ -444,7 +444,10 @@ def _visit_part(self, part, spacing, leaf):
                         should_be_indentation = n.indentation
 
                         self._last_indentation_tos = n
-                        if n == node:
+                        # A backslash in the same prefix can introduce a new
+                        # node below the previous indentation stack. In that
+                        # case, stop at the root instead of walking past it.
+                        if n == node or n.parent is None:
                             break
                         n = n.parent
 

test/test_pep8.py

@@ -1,4 +1,5 @@
 import parso
+import pytest
 
 
 def issues(code):
@@ -40,3 +41,14 @@ def test_shebang():
     assert not issues('#!\n')
     assert not issues('#!/foo\n')
     assert not issues('#! python\n')
+
+
+@pytest.mark.parametrize('newline', ['\n', '\r\n', '\r'])
+@pytest.mark.parametrize('backslashes', [1, 2])
+@pytest.mark.parametrize('comment', ['#', '# comment'])
+def test_comment_after_backslash(newline, backslashes, comment):
+    prefix = ('\\' + newline) * backslashes
+    found = issues(prefix + comment)
+    assert [(issue.code, issue.start_pos) for issue in found] == [
+        (292, (backslashes + 1, len(comment))),
+    ]