CVE-2026-73471: Gstreamer1-plugins-good: gstreamer: out-of-bounds read in avidemux nikon metadata tag parsing

A flaw was found in GStreamer gst-plugins-good (avidemux). In the Nikon metadata tag parser, a pointer can be dereferenced without verifying that data remains in the buffer, causing out-of-bounds reads. Because avidemux is auto-plugged by playbin, decodebin, and gst-discoverer, opening or previewing a crafted AVI is sufficient to trigger the issue. Fixed upstream in gst-plugins-good 1.28.6 (GStreamer-SA-2026-0072).