Fix ECDHE-PSK premaster secret derivation.

The original implementation used the wrong premaster secret; it uses
psk || other_secret rather than other_secret || psk. Fix the
implementation to get it in the right order.

See BoringSSL change https://boringssl-review.googlesource.com/#/c/2052/

Bug: 18147456

(cherry picked from commit d267f08e9ba3894f091344b2a4e3e55ad2498c24)

Change-Id: Ia6576c4c0e28722e66422e24ed0373a86d00efce
3 files changed