[AssetLib] Fix crash in Web editor.

Add EditorAssetLibrary::is_available which always returns false in the
Web editor and use it in EditorNode for detection.

editor/plugins/asset_library_editor_plugin.cpp

@@ -32,6 +32,7 @@
 
 #include "core/input/input.h"
 #include "core/io/json.h"
+#include "core/io/stream_peer_ssl.h"
 #include "core/os/keyboard.h"
 #include "core/version.h"
 #include "editor/editor_file_dialog.h"
@@ -1588,6 +1589,16 @@ EditorAssetLibrary::EditorAssetLibrary(bool p_templates_only) {
 
 ///////
 
+bool AssetLibraryEditorPlugin::is_available() {
+#ifdef JAVASCRIPT_ENABLED
+	// Asset Library can't work on Web editor for now as most assets are sourced
+	// directly from GitHub which does not set CORS.
+	return false;
+#else
+	return StreamPeerSSL::is_available();
+#endif
+}
+
 void AssetLibraryEditorPlugin::make_visible(bool p_visible) {
 	if (p_visible) {
 		addon_library->show();