Silence some 'variable may be used uninitialized' compiler warnings

when compiling with -DG_DISABLE_ASSERT

ext/alsa/gstalsa.c

@@ -271,6 +271,7 @@ caps_add_channel_configuration (GstCaps * caps,
           channel_mask = CHANNEL_MASK_7_1;
           break;
         default:
+          channel_mask = 0;
           g_assert_not_reached ();
           break;
       }

tests/examples/playback/playback-test.c

@@ -1611,7 +1611,7 @@ colorbalance_value_changed (GtkRange * range, PlaybackApp * app)
   else if (range == GTK_RANGE (app->saturation_scale))
     label = "SATURATION";
   else
-    g_assert_not_reached ();
+    g_return_if_reached ();
 
   val = gtk_range_get_value (range);