avidemux: Avoid potential double-free
avidemux: Avoid potential double-free

stream->name was being freed (without being NULL-ed) before we were certain it
would be set again.

CID #1456071

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/582>

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