tests: avoid assert in adder test if no audio sources work

The array of factories should not contain a NULL element at the end
since the number of arguments is determined via G_N_ELEMENTS and the
NULL will be used as an argument to gst_element_factory_make() if
the other sources in the list weren't usable.

tests/check/elements/adder.c

@@ -587,8 +587,7 @@ GST_START_TEST (test_live_seeking)
   /* don't use autoaudiosrc, as then we can't set anything here */
   const gchar *audio_src_factories[] = {
     "alsasrc",
-    "pulseaudiosrc",
-    NULL,
+    "pulseaudiosrc"
   };
 
   GST_INFO ("preparing test");