fix LGTM

Signed-off-by: vaibhav-dahiya <[email protected]>

Author: vdahiya12

sonic-ycabled/ycable/ycable_utilities/y_cable_helper.py

@@ -4039,4 +4039,7 @@ def join(self):
         helper_logger.log_info("stopped all thread")
 
         if self.exc is not None:
+
+            assert self._thread is not None # prevent illegal false positive LGTM
+
             raise self.exc