Fix NameError and socket leak in TCPClient._create_stream error path

[bysiber]

In TCPClient._create_stream, if IOStream() constructor raises OSError, the except handler references stream which was never assigned:

try:
    stream = IOStream(socket_obj, max_buffer_size=max_buffer_size)
except OSError as e:
    fu = Future()
    fu.set_exception(e)
    return stream, fu  # NameError: 'stream' is not defined

This causes a NameError that masks the original OSError, making it impossible to diagnose the root cause from the traceback.

Additionally the socket object would be leaked since it's never closed in the error path.

Fixed by returning the socket_obj instead of the undefined stream variable, and closing the socket before returning.