`Frame::copy_from_raw_parts` can lead to segfault without `unsafe`

fn Frame::copy_from_raw_parts() is a safe API that can take a
raw pointer and dereference it. It is possible to read arbitrary memory address
with an arbitrarily fed pointer. This allows the safe API to access & read
arbitrary address in memory. Feeding an invalid memory address pointer to
the API may also cause the program to segfault.

The flaw was corrected in https://github.com/rust-av/rust-av/pull/137,
by removing the API fn Frame::copy_from_raw_parts().