Use order not degree to calculate a buffer size in ecdsatest

Otherwise this can result in an incorrect calculation of the maximum
encoded integer length, meaning an insufficient buffer size is allocated.

Thanks to Billy Brumley for helping to track this down.

Fixes #8209
Reviewed-by: NNicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8237)

(cherry picked from commit 9fc8f18f59f4a4c853466dca64a23b8af681bf1c)

test/ecdsatest.c

@@ -223,7 +223,7 @@ static int test_builtin(void)
     const BIGNUM *sig_r, *sig_s;
     BIGNUM *modified_r = NULL, *modified_s = NULL;
     BIGNUM *unmodified_r = NULL, *unmodified_s = NULL;
-    unsigned int sig_len, degree, r_len, s_len, bn_len, buf_len;
+    unsigned int sig_len, order, r_len, s_len, bn_len, buf_len;
     int nid, ret = 0;
 
     /* fill digest values with some random data */
@@ -251,7 +251,7 @@ static int test_builtin(void)
                 || !TEST_true(EC_KEY_set_group(eckey, group)))
             goto builtin_err;
         EC_GROUP_free(group);
-        degree = EC_GROUP_get_degree(EC_KEY_get0_group(eckey));
+        order = EC_GROUP_order_bits(EC_KEY_get0_group(eckey));
 
         TEST_info("testing %s", OBJ_nid2sn(nid));
 
@@ -316,7 +316,7 @@ static int test_builtin(void)
         /* Store the two BIGNUMs in raw_buf. */
         r_len = BN_num_bytes(sig_r);
         s_len = BN_num_bytes(sig_s);
-        bn_len = (degree + 7) / 8;
+        bn_len = (order + 7) / 8;
         if (!TEST_false(r_len > bn_len)
                 || !TEST_false(s_len > bn_len))
             goto builtin_err;