Work around a bug user auth banner message generated by cryptlib

This commit works around the inclusion of an extra NUL byte in
user auth banner messages generated by the cryptlib library.
Thanks go to GitHub user mmayomoar for reporting this issue and
suggesting a fix.

asyncssh/connection.py

@@ -2501,6 +2501,13 @@ def _process_userauth_banner(self, _pkttype: int, _pktid: int,
 
         msg_bytes = packet.get_string()
         lang_bytes = packet.get_string()
+
+        # Work around an extra NUL byte appearing in the user
+        # auth banner message in some versions of cryptlib
+        if b'cryptlib' in self._server_version and \
+                packet.get_remaining_payload() == b'\0': # pragma: no cover
+            packet.get_byte()
+
         packet.check_end()
 
         try: