fix gen_copy when length is 0
fix gen_copy when length is 0

add a gen_slice test with empty slice to trigger that.
If we're at index 4 in a 7-length buffer and we have 3 to write, we'll write at index 4, 5, 6, the buffer is not too small.

Signed-off-by: Marc-Antoine Perennou <07f76cf0511c79b361712839686f3cee8c75791c@Perennou.com>

Geal/cookie-factory