imagefreeze: fix use-after-free on seek event
imagefreeze: fix use-after-free on seek event

Get seqnum before unreffing the seek event.

https://bugzilla.gnome.org/show_bug.cgi?id=784486

StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good