Fix kTwoPrimeKey and kSixPrimeKey samples in rsa_test.

They weren't valid DER. Some lengths were encoded with one more byte
than necessary.

Change-Id: I94c8c525ade835fdeca115af98ab7e5910d2aeb2
Reviewed-on: https://boringssl-review.googlesource.com/5265
Reviewed-by: Adam Langley <agl@google.com>