misc: Fix a small pile of warnings found with diagnostics turned way up

* possible null pointer dereferences
 * non-annotated fallthroughs in case statements

Change-Id: I2332e0e0a4203ceca0999291ead112b8ab94c613

apps/plugins/imageviewer/imageviewer.c

@@ -406,6 +406,7 @@ static int ask_and_get_audio_buffer(const char *filename)
                 break;
 #endif
 #endif
+            /* Intentional fallthrough */
             case IMGVIEW_MENU:
                 return PLUGIN_OK;