rtph264pay: Only mark the last fragment of an AU
rtph264pay: Only mark the last fragment of an AU

Commit 4add820cce278213ede3d5fce427ea92e0619b6f removed the check for
the end of fragmentation. As a result, all fragments of an AU's last
NALU were marked.

Potential fix for https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/issues/540

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