browser-plugin: Fix callback for "onchange"

In the case that calling the listener fails, "result" may be uninitialized.
Sending NPAPI uninitialized memory is never a good idea.

https://bugzilla.gnome.org/show_bug.cgi?id=668517

browser-plugin/browser-plugin.c

@@ -286,7 +286,7 @@ on_shell_signal (GDBusProxy *proxy,
       gint32 status;
       gchar *error;
       NPVariant args[3];
-      NPVariant result;
+      NPVariant result = { NPVariantType_Void };
 
       g_variant_get (parameters, "(sis)", &uuid, &status, &error);
       STRINGZ_TO_NPVARIANT (uuid, args[0]);