net: phylink: correctly validate returned PCS in phylink_inband_caps

In the Linux kernel, the following vulnerability has been resolved: net: phylink: correctly validate returned PCS in phylink_inband_caps In phylink_inband_caps(), the PCS returned by mac_select_pcs is only checked if NULL but mac_select_pcs can also return an error pointer. This can cause a kernel panic as phylink_pcs_inband_caps() only checks if passed PCS is not NULL and directly dereference ops from the phylink_pcs struct. Use the IS_ERR_OR_NULL macro to address both case where the returned PCS can be NULL or an error pointer and prevent a kernel panic.