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 ras_core_get_utc_second_timestamp()

ras_core_get_utc_second_timestamp() 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 ras_core in the conditional statement before calling

the sys_fn callback. However, when the condition fails, the function

prints an error message using ras_core->dev.

If ras_core is NULL, this can lead to a potential NULL pointer

dereference when accessing ras_core->dev.

Add an early NULL check for ras_core 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.