tests: fix leak in splitmux test

https://bugzilla.gnome.org/show_bug.cgi?id=781025
diff --git a/tests/check/elements/splitmux.c b/tests/check/elements/splitmux.c
index 6c4beb8..c6d2ef1 100644
--- a/tests/check/elements/splitmux.c
+++ b/tests/check/elements/splitmux.c
@@ -507,6 +507,7 @@
 
   gst_element_set_state (pipeline, GST_STATE_NULL);
 
+  g_clear_object (&appsrc);
   g_clear_object (&bus);
   g_clear_object (&pipeline);