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.

Fixes: 13c91b5b ("drm/amd/ras: Add rascore unified interface function")
Cc: YiPeng Chai <YiPeng.Chai@amd.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Tao Zhou <tao.zhou1@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: YiPeng Chai <YiPeng.Chai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>