CVE-2024-42224: Improper Check for Unusual or Exceptional Conditions

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: mv88e6xxx: Correct check for empty list

Since commit a3c53be55c95 ("net: dsa: mv88e6xxx: Support multiple MDIO busses") mv88e6xxx_default_mdio_bus() has checked that the return value of list_first_entry() is non-NULL.

This appears to be intended to guard against the list chip->mdios being empty. However, it is not the correct check as the implementation of list_first_entry is not designed to return NULL for empty lists.

Instead, use list_first_entry_or_null() which does return NULL if the list is empty.

Flagged by Smatch. Compile tested only.