Since this function is called with packets on the stack, trying to free
them makes no sense. Instead, it should unref (which is what
`av_interleaved_write_frame` does anyway, rather than freeing).
Also, the calling code tried unreffing the packet a second time, even
after it was "freed" by the callee in the failure case - and after
ownership was taken over by `av_interleaved_write_frame` in the
successful case. Both of these cases were wrong.