CVE-2015-7558: librsvg DoS via Cyclic References

The vulnerability (CVE-2015-7558) stems from improper cycle detection when resolving SVG element references. The fix in commit a51919f7e1ca... introduced recursion depth tracking in these functions to prevent infinite loops. Prior to this patch, rsvg_acquire_node and rsvg_acquire_node_of_type recursively resolved hrefs without limiting recursion depth, leading to stack overflow when cyclic references were present. The functions' role in reference resolution and the explicit addition of depth checks in the patch confirm their vulnerability.