fix ras err_data null pointer issue in amdgpu_ras.c

Fixes: 8cc0f566 ("drm/amdgpu: Support multiple error query modes")
Signed-off-by: Yang Wang <kevinyang.wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>