Guard against possible missing cloud filter export

It's possible that a legacy version of `cldapi.dll` is present on older
systems where it's not officially supported, which may have a different
set of exports. Guard against this by explicitly checking if the export
we desire is present.

Author: zao

ui_api.cpp

@@ -681,12 +681,12 @@ struct CloudProviderLibrary {
 		FreeLibrary(cldLib);
 	}
 
-	bool Loaded() const { return cldLib != nullptr; }
+	bool Loaded() const { return cldLib != nullptr && CfGetSyncRootInfoByPath != nullptr; }
 
 	CloudProviderLibrary(CloudProviderLibrary const&) = delete;
 	CloudProviderLibrary& operator = (CloudProviderLibrary const&) = delete;
 
-	decltype (&::CfGetSyncRootInfoByPath) CfGetSyncRootInfoByPath;
+	decltype (&::CfGetSyncRootInfoByPath) CfGetSyncRootInfoByPath{};
 
 	HMODULE cldLib{};
 };