qtmux: Don't forget to update pad->last_buf
qtmux: Don't forget to update pad->last_buf

buf is the current pad->last_buf value. If ever it gets copied/unreffed,
we need to make sure to write back the new  pointer to the last_buf
variable.

Fixes using wrong pointer values in the case of decrasing DTS value

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