CVE-2026-53315: drm/amd/ras: Fix NULL deref in ras_core_get_utc_second_timestamp()

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

drm/amd/ras: Fix NULL deref in rascoregetutcsecondtimestamp()

rascoregetutcsecondtimestamp() retrieves the current UTC timestamp (in seconds since the Unix epoch) through a platform-specific RAS system callback and is used for timestamping RAS error events.

The function checks rascore in the conditional statement before calling the sysfn callback. However, when the condition fails, the function prints an error message using rascore->dev.

If rascore is NULL, this can lead to a potential NULL pointer dereference when accessing rascore->dev.

Add an early NULL check for rascore at the beginning of the function and return 0 when the pointer is not valid. This prevents the dereference and makes the control flow clearer.