drm/amdgpu: Handle job is NULL use case in amdgpu_device_gpu_recover

This should be checked at all places job is accessed.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>