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.

Fixes: ece19502 ("net: phy: micrel: 1588 support for LAN8814 phy")
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240329061631.33199-1-amishin@t-argos.ruSigned-off-by: Jakub Kicinski <kuba@kernel.org>