drm/amdgpu: Check if fru_addr is not NULL (v2)

Always check if fru_addr is not NULL. This commit also fixes a "smatch"
warning.

v2: Add a Fixes tag.

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Dan Carpenter <error27@gmail.com>
Cc: kernel test robot <lkp@intel.com>
Cc: AMD Graphics <amd-gfx@lists.freedesktop.org>
Fixes: afbe5d1e ("drm/amdgpu: Bug-fix: Reading I2C FRU data on newer ASICs")
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Kent Russell <kent.russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>