Premaster secret handling fixes

From BoringSSL
- Send an alert when the client key exchange isn't correctly formatted.
- Reject overly short RSA ciphertexts to avoid a (benign) out-of-bounds memory access.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
Build fixes

Various build fixes, mostly uncovered by clang's unused-const-variable
and unused-function errors.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>
(cherry picked from commit 0e1c318ece3c82e96ae95a34a1badf58198d6b28)
Revert "RT3425: constant-time evp_enc"

Causes more problems than it fixes: even though error codes
are not part of the stable API, several users rely on the
specific error code, and the change breaks them. Conversely,
we don't have any concrete use-cases for constant-time behaviour here.

This reverts commit 4aac102f75b517bdb56b1bcfd0a856052d559f6e.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Add a comment noting the padding oracle.

Reviewed-by: Andy Polyakov <appro@openssl.org>