flvmux: Fix possible crash on GST_ITERATOR_RESYNC
flvmux: Fix possible crash on GST_ITERATOR_RESYNC

Wrong pointer type

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/696>

GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good