box: Free children of binary coded box by its destructor.
lib: Add NULL check for lsmash_memdup().
lib: Fix size and data check for lsmash_create_box().
lib: Add lsmash_add_box_ex().

This function tries to expand into struct format for the internal
references within the L-SMASH library when adding known box.
If the adding box is unknown, do the similar way of lsmash_add_box().