celtdepay: calculate size correctly
celtdepay: calculate size correctly

The summation was done wrong, causing the de-payloader to exit its loop too
early, before all frames are processed.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=674472

knuesel/gst-plugins-good,knuesel/gst-plugins-good,knuesel/gst-plugins-good,knuesel/gst-plugins-good