Minor bugfix: Add a NULL check for dh
Replace our consttime_memcmp with OpenSSLs builtin CRYPTO_memcmp

CRYPTO_memcmp is also constant time (as opposed to OPENSSL_memcmp).
Furthermore our consttime_memcmp was buggy.
Merge remote-tracking branch 'upstream/master'
Perform a length check before CRYPTO_memcmp to avoid out-of-bounds reads
Fix merge conflict
Merge pull request #10 from d0/master

Perform a length check before CRYPTO_memcmp to avoid out-of-bounds reads