drm/amdgpu: Fix potential null pointer derefernce

The amdgpu_ras_get_context may return NULL if device
not support ras feature, so add check before using.
Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_device.c

@@ -5438,7 +5438,8 @@ int amdgpu_device_gpu_recover(struct amdgpu_device *adev,
 	 * Flush RAM to disk so that after reboot
 	 * the user can read log and see why the system rebooted.
 	 */
-	if (need_emergency_restart && amdgpu_ras_get_context(adev)->reboot) {
+	if (need_emergency_restart && amdgpu_ras_get_context(adev) &&
+		amdgpu_ras_get_context(adev)->reboot) {
 		DRM_WARN("Emergency reboot.");
 
 		ksys_sync_helper();