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.