ssl/t1_enc.c: check EVP_MD_CTX_copy return value.

PR: 3201

(Imported from upstream's 8d08627c94)

ssl/t1_enc.c

@@ -1221,7 +1221,8 @@ int tls1_mac(SSL *ssl, unsigned char *md, int send)
 		}
 		else
 		{
-			EVP_MD_CTX_copy(&hmac,hash);
+			if (!EVP_MD_CTX_copy(&hmac,hash))
+				return -1;
 			mac_ctx = &hmac;
 		}