allow nullable target for ghost pads

gst/gst_ghost_pad.go

@@ -123,8 +123,14 @@ func (g *GhostPad) GetTarget() *Pad {
 // SetTarget sets the new target of the ghostpad gpad. Any existing target is unlinked and links to the new target are
 // established. if newtarget is nil the target will be cleared.
 func (g *GhostPad) SetTarget(target *Pad) bool {
+	var ctarget *C.GstPad
+
+	if target != nil {
+		ctarget = target.Instance()
+	}
+
 	return gobool(C.gst_ghost_pad_set_target(
-		g.Instance(), target.Instance(),
+		g.Instance(), ctarget,
 	))
 }