opusenc: Unref pad template caps after usage

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734517
sbcenc: Unref output caps in error cases

gst_caps_intersect() returns a reference to a caps object which in
the normal case is consumed, but in error cases must be unreferenced.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734521