Fix asserts in CRYPTO_ctr128_encrypt_ctr32.

NULL in and out are legal in the degenerate case when len is 0.

Change-Id: Ibf0600a4f635a03103b1ae914918fdcf23a75a39
Reviewed-on: https://boringssl-review.googlesource.com/4705
Reviewed-by: Adam Langley <agl@google.com>