Add a test for bad IDNA in ssl server_hostname (#1997)

See discussion:
  #1992 (comment)

Author: njsmith

Lib/test/test_ssl.py

@@ -1393,6 +1393,16 @@ def test_subclass(self):
                 # For compatibility
                 self.assertEqual(cm.exception.errno, ssl.SSL_ERROR_WANT_READ)
 
+    def test_bad_idna_in_server_hostname(self):
+        # Note: this test is testing some code that probably shouldn't exist
+        # in the first place, so if it starts failing at some point because
+        # you made the ssl module stop doing IDNA decoding then please feel
+        # free to remove it. The test was mainly added because this case used
+        # to cause memory corruption (see bpo-30594).
+        ctx = ssl.create_default_context()
+        with self.assertRaises(UnicodeError):
+            ctx.wrap_bio(ssl.MemoryBIO(), ssl.MemoryBIO(),
+                         server_hostname="xn--.com")
 
 class MemoryBIOTests(unittest.TestCase):