`**strong***em***strong**` incorrect result

[justanotheranonymoususer]

Describe the bug
This: **strong***em***strong**

To Reproduce

import markdown2

markdown_text = """
**strong***em***strong**
"""

html_output = markdown2.markdown(markdown_text)

print(html_output)

Expected behavior
Expected:

<p><strong>strong</strong><em>em</em><strong>strong</strong></p>

Actual:

<p><strong>strong<em></strong>em<strong></em>strong</strong></p>

Debug info
Version of library being used: 8d50892

[justanotheranonymoususer]

Other mismatches that I saw, not sure if they warrant new issues since they're more specific. Let me know if you prefer new issues.

** *em*

GitHub and other flavors: ** em
markdown2: <em>* *em</em> or rendered: * *em

****bold****

GitHub and other flavors: bold
markdown2: <strong><em>*bold</em></strong>* or rendered: *bold*

*em***bold**

GitHub and other flavors: embold
markdown2: <em>em<strong></em>bold</strong> - invalid markup

**bold****

GitHub and other flavors: bold**
markdown2: <strong>bold<em></strong></em> - bold (invalid markup)