Fix crypto/bytestring test for too long lengths.

kData5 was meant to test lengths that are too long, but the input
gets rejected earlier for not using short-form encoding. Switch it to
testing a badly encoded element of length 128, the shortest element that
uses long-form encoding.

Change-Id: I35f4df89bfa7a681698eda569c525b5871288487
Reviewed-on: https://boringssl-review.googlesource.com/5264
Reviewed-by: Adam Langley <agl@google.com>