drm/amdgpu: Fix potential NULL dereference

Fix potential NULL dereference, in the case when "man", the resource manager
might be NULL, when/if we print debug information.

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: AMD Graphics <amd-gfx@lists.freedesktop.org>
Cc: Dan Carpenter <error27@gmail.com>
Cc: kernel test robot <lkp@intel.com>
Fixes: 7554886d ("drm/amdgpu: Fix size validation for non-exclusive domains (v4)")
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -470,6 +470,7 @@ static bool amdgpu_bo_validate_size(struct amdgpu_device *adev,
 	return true;
 
 fail:
+	if (man)
 		DRM_DEBUG("BO size %lu > total memory in domain: %llu\n", size,
 			  man->size);
 	return false;