browser-plugin: Fix Opera support

Opera now tries to call NP_SetWindow without trying to check
if it's a NULL pointer or not.

https://bugzilla.gnome.org/show_bug.cgi?id=672192

browser-plugin/browser-plugin.c

@@ -163,6 +163,7 @@ NP_Initialize(NPNetscapeFuncs *pfuncs, NPPluginFuncs *plugin)
   plugin->newp = NPP_New;
   plugin->destroy = NPP_Destroy;
   plugin->getvalue = NPP_GetValue;
+  plugin->setwindow = NPP_SetWindow;
 
   return NPERR_NO_ERROR;
 }
@@ -969,3 +970,12 @@ NPP_GetValue(NPP          instance,
 
   return NPERR_NO_ERROR;
 }
+
+/* Opera tries to call NPP_SetWindow without checking the
+ * NULL pointer beforehand. */
+NPError
+NPP_SetWindow(NPP          instance,
+              NPWindow    *window)
+{
+  return NPERR_NO_ERROR;
+}