Rather than use an internal function in a test (which would need an
OPENSSL_EXPORT to work in a shared-library build), this change corrupts
the secret key directly.
Change-Id: Iee501910b23a0affaa0639dcc773d6ea2d0c5a82
Reviewed-on: https://boringssl-review.googlesource.com/7780
Reviewed-by: Matt Braithwaite <mab@google.com>
Reviewed-by: Adam Langley <agl@google.com>