Fix EVP_PKEY_assign_DH.

Or rather fix in so far as that call will always fail now, rather than
mix up EC and DH EVP_PKEY. We don't implement EVP_PKEY_DH.

Change-Id: I752978f3440b59d963b5c13f2349284d7d799182
Reviewed-on: https://boringssl-review.googlesource.com/4567
Reviewed-by: Adam Langley <agl@google.com>

crypto/evp/evp.c

@@ -323,7 +323,7 @@ int EVP_PKEY_set1_DH(EVP_PKEY *pkey, DH *key) {
 }
 
 int EVP_PKEY_assign_DH(EVP_PKEY *pkey, DH *key) {
-  return EVP_PKEY_assign(pkey, EVP_PKEY_EC, key);
+  return EVP_PKEY_assign(pkey, EVP_PKEY_DH, key);
 }
 
 DH *EVP_PKEY_get1_DH(EVP_PKEY *pkey) {