[security] bpo-13617: Reject embedded null characters in wchar* strings. (#2302)

Based on patch by Victor Stinner.

Add private C API function _PyUnicode_AsUnicode() which is similar to
PyUnicode_AsUnicode(), but checks for null characters.

Lib/test/test_grp.py

@@ -50,6 +50,8 @@ class GroupDatabaseTestCase(unittest.TestCase):
         self.assertRaises(TypeError, grp.getgrgid)
         self.assertRaises(TypeError, grp.getgrnam)
         self.assertRaises(TypeError, grp.getgrall, 42)
+        # embedded null character
+        self.assertRaises(ValueError, grp.getgrnam, 'a\x00b')
 
         # try to get some errors
         bynames = {}