Ensure EC private keys retain leading zeros

RFC 5915 requires the use of the I2OSP primitive as defined in RFC 3447
for encoding ECPrivateKey. Fix this and add a test.

See also upstream's 30cd4ff294252c4b6a4b69cbef6a5b4117705d22, though it mixes
up degree and order.

Change-Id: I81ba14da3c8d69e3799422c669fab7f16956f322
Reviewed-on: https://boringssl-review.googlesource.com/4469
Reviewed-by: Adam Langley <agl@google.com>
2 files changed