Adding in nil checks for completion handlers of Streaming Media Manager.
Adding in nil checks for completion handlers of Streaming Media Manager.

FordDev/sdl_ios,APCVSRepo/sdl_ios,APCVSRepo/sdl_ios,APCVSRepo/sdl_ios,smartdevicelink/sdl_ios,FordDev/sdl_ios,FordDev/sdl_ios,kshala-ford/sdl_ios,smartdevicelink/sdl_ios,kshala-ford/sdl_ios,FordDev/sdl_ios,smartdevicelink/sdl_ios,kshala-ford/sdl_ios,smartdevicelink/sdl_ios,davidswi/sdl_ios,APCVSRepo/sdl_ios,kshala-ford/sdl_ios,FordDev/sdl_ios,davidswi/sdl_ios,kshala-ford/sdl_ios,APCVSRepo/sdl_ios,smartdevicelink/sdl_ios,davidswi/sdl_ios,davidswi/sdl_ios,davidswi/sdl_ios