SeedableRng::seed_from_u64 panics if the Seed type's length is not a multiple of 4

[sfackler]

If a SeedableRng implementation uses a seed type with a length that isn't divisible by 4, the final chunk of the seed buffer will not be 4 bytes, so the copy_from_slice call will panic: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=2e5eea3830867e62fe8a47f02721a60a.

[dhardy]

Fix published (rand_core 0.6.1)