Heap corruption via signed Base64 length overflow

Out-of-bounds writes in GLib's Base64 encoder on Red Hat Enterprise Linux 6 through 10 allow attackers to corrupt application memory. g_base64_encode() stores the length from g_base64_encode_step() in a signed gint; overflow makes it negative, and g_base64_encode_close() then performs pointer arithmetic that writes before the heap buffer. Reachability requires user interaction and an application that encodes attacker-controlled or extremely large input; corruption can crash the process or alter memory.