Fix memory leak on iOS shared core helper

[r-rebacz]

Description

This PR fixes memory leak in one of iOS shared core helpers, whenever user defaults do not contain messages object.

How to reproduce

• linphone lib integrated with iOS app.
• Shared linphone core initialised with configuration:

       let config = Config.newForSharedCore(
                   appGroupId: ...,
                   configFilename: ...,
                   factoryConfigFilename: ...
       )

        do {
            core = try Factory.Instance.createSharedCoreWithConfig(
                config: config,
                systemContext: nil,
                appGroupId: ...,
                mainCore: true
            )
        }  catch {
            return nil
        }

• Run the app.
• Put a breakpoint right after core initialization. Open Xcodes object graph debugger. Turn on "leaked objects" filter.

How fix works

I think code is self explanatory :) It's all about missing release() call.