socket must be None, not shell_socket for default shell

[minrk]

process_shell_message has

assert socket is None

for the default subshell, so passing shell_socket here results in:

socket = self.shell_socket
...
assert socket is None
socket = self.shell_socket

[minrk]

alternative change would be to update the assert to assert socket in {self.shell_socket, None}, but this seems to match other calls to shell_main

[ianthomas23]

Thanks @minrk