sd-json: fix sd_json_variant_unsigned() dispatching to wrong accessor for references

sd-json: fix sd_json_variant_unsigned() dispatching to wrong accessor for references

sd_json_variant_unsigned() incorrectly calls sd_json_variant_integer()
for reference-type variants instead of recursing to itself. This silently
returns 0 for unsigned values in the range INT64_MAX+1 through
UINT64_MAX, since sd_json_variant_integer() cannot represent them.

The sibling functions sd_json_variant_integer() and
sd_json_variant_real() correctly recurse to themselves.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>