net: phy: micrel: Fix potential null pointer dereference

In the Linux kernel, the following vulnerability has been resolved: net: phy: micrel: Fix potential null pointer dereference In lan8814_get_sig_rx() and lan8814_get_sig_tx() ptp_parse_header() may return NULL as ptp_header due to abnormal packet type or corrupted packet. Fix this bug by adding ptp_header check. Found by Linux Verification Center (linuxtesting.org) with SVACE.