See upstream's bd891f098bdfcaa285c073ce556d0f5e27ec3a10. It honestly seems
kinda dumb for a client to do this, but apparently the spec allows this.
Judging by code inspection, OpenSSL 1.0.1 also allowed this, so this avoids a
behavior change when switching from 1.0.1 to BoringSSL.
Add a test for this, which revealed that, unlike upstream's version, this
actually works with ecdh_auto since tls1_get_shared_curve also needs updating.
(To be mentioned in newsletter.)
Change-Id: Ie622700f17835965457034393b90f346740cfca8
Reviewed-on: https://boringssl-review.googlesource.com/4464
Reviewed-by: Adam Langley <agl@google.com>
55a436497f