Fix invalid assert in CRYPTO_ctr128_encrypt.

As with CRYPTO_ctr128_encrypt_ctr32, NULL in and out are legal in the
degenerate case when len is 0. This fixes one of the two failures on the bots.

Change-Id: If6016dfc3963d9c06c849fc8eba9908556f66666
Reviewed-on: https://boringssl-review.googlesource.com/4721
Reviewed-by: Adam Langley <agl@google.com>