prevent very large headline prefixes

for example: `<h9999999>` could crash the conversion.

fixes #143

markdownify/__init__.py

@@ -315,6 +315,9 @@ class MarkdownConverter(object):
         if convert_as_inline:
             return text
 
+        # prevent MemoryErrors in case of very large n
+        n = max(1, min(6, n))
+
         style = self.options['heading_style'].lower()
         text = text.strip()
         if style == UNDERLINED and n <= 2:

tests/test_conversions.py

@@ -133,6 +133,7 @@ def test_hn():
     assert md('<h4>Hello</h4>') == '\n#### Hello\n\n'
     assert md('<h5>Hello</h5>') == '\n##### Hello\n\n'
     assert md('<h6>Hello</h6>') == '\n###### Hello\n\n'
+    assert md('<h10>Hello</h10>') == md('<h6>Hello</h6>')
 
 
 def test_hn_chained():