deinterlace: fix caps leak
deinterlace: fix caps leak

The caps returned by gst_pad_get_current_caps() was never unreffed when
not early returning.

Fix a leak with the elements/deinterlace test.

https://bugzilla.gnome.org/show_bug.cgi?id=766558

Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good,Kurento/gst-plugins-good