Minor Fix in Server: fix error "has no attribute 'last_read_time'"

[da-h]

Description

I noticed that the last tests threw the following error when using the -use_frontend_client_polling option:

 ERROR:tornado.application:Exception in callback <bound method SocketWrapper.socket_wrap_monitor_thread of <visdom.server.handlers.socket_handlers.SocketWrapper object at 0x7efec864b670>>
Traceback (most recent call last):
  File "/home/runner/.local/lib/python3.10/site-packages/tornado/ioloop.py", line 921, in _run
    val = self.callback()
  File "/home/runner/work/visdom/visdom/py/visdom/server/handlers/socket_handlers.py", line 336, in socket_wrap_monitor_thread
    if time.time() - sub.last_read_time > MAX_SOCKET_WAIT:

(e.g. here)

This PR is a simple fix for this error, by checking if the variable exists.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactor or cleanup (changes to existing code for improved readability or performance)

Checklist:

• [ ] I adapted the version number under py/visdom/VERSION according to Semantic Versioning
• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.