img: fix apparent double-free in img_svg_render() failure path

img: fix apparent double-free in img_svg_render() failure path

img_svg_render() calls g_object_unref() on the RsvgHandle in its
error path, but the handle is owned by the shared lab_img_data
struct and will be double-freed later by lab_img_destroy().

The double-free was introduced when img_svg_load() was split from
img_svg_render(). The g_object_unref() should have been removed from
img_svg_render() but was missed.

Fixes: 16dbdc64e58d66011bbf319b92de844dab0ca8d9
("ssd: rework titlebar button rendering")