Fix AssertionError on "..." outside of a continuation prompt

Thanks @Zac-HD for the report and fix!

Fixes #69

blacken_docs.py

@@ -127,8 +127,7 @@ def finish_fragment() -> None:
         for line in match['code'].splitlines():
             orig_line, line = line, line.lstrip()
             continuation_match = PYCON_CONTINUATION_RE.match(line)
-            if continuation_match:
-                assert fragment is not None
+            if continuation_match and fragment is not None:
                 fragment += line[continuation_match.end():] + '\n'
             else:
                 finish_fragment()

tests/blacken_docs_test.py

@@ -672,3 +672,16 @@ def test_format_src_rst_pycon_preserves_output_indentation():
     )
     after, _ = blacken_docs.format_str(before, BLACK_MODE)
     assert after == before
+
+
+def test_format_src_rst_pycon_elided_traceback():
+    before = (
+        '.. code-block:: pycon\n'
+        '\n'
+        '    >>> 1 / 0\n'
+        '    Traceback (most recent call last):\n'
+        '      ...\n'
+        '    ZeroDivisionError: division by zero\n'
+    )
+    after, _ = blacken_docs.format_str(before, BLACK_MODE)
+    assert after == before