Avoid double svg error reporting and segfaults

Avoid double svg error reporting and segfaults

* src/image.c (svg_load_image): Use g_error_free directly instead
of the helder g_clear_error (since we're only calling it with
non-nil values).
(svg_load_image): Avoid segfault and double reporting errors.
* test/manual/image-tests.el (image-tests-load-image/svg-too-big)
(image-tests-load-image/svg-invalid): Test it (bug#57755).