media_utils: fix segfault when stopping a recording

[Megamouse]

• use avio_closep instead of avio_close in order to properly reset the pointer
• don't use avcodec_close. use avcodec_free_context as mentioned in the avcodec_close description
• don't use avformat_free_context if we already used avformat_close_input which sets the pointer to null anyway

fixes #15039