HuC authentication is called even if HuC firmware is not present in the
system, leading to NULL ptr dereference on not allocated gem_object.
Let's avoid trying to authenticate HuC if its firmware is not loaded
successfully.
Fixes: dac84a388528 ("drm/i915/huc: Support HuC authentication")
v2: Check inside the auth function, split the assert (Michał)
v3: Oops, hit send before compiling, s/huc_fw/huc->fw