scrypt: free the MD reference correctly.

The code was calling EVP_MD_meth_free which is incorrect.  It should call
EVP_MD_free.  It happened to work but by luck rather than design.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10973)

providers/implementations/kdfs/scrypt.c

@@ -75,7 +75,7 @@ static void kdf_scrypt_free(void *vctx)
     KDF_SCRYPT *ctx = (KDF_SCRYPT *)vctx;
 
     if (ctx != NULL) {
-        EVP_MD_meth_free(ctx->sha256);
+        EVP_MD_free(ctx->sha256);
         kdf_scrypt_reset(ctx);
         OPENSSL_free(ctx);
     }