Regression: Hang with code-friendly

Version: https://github.com/trentm/python-markdown2/commit/f18725f0851afad72e4524c3275f3fb5c9a35d26

```py
import markdown2

# This input causes the code to hang infinitely.
markdown_text = R"""
a_b **x***y* c_d
"""
print(markdown2.markdown(markdown_text, extras={'code-friendly': None}))
```