Support partial export from mbedtls_ecp_keypair

Sometimes you don't need to have all the parts of a key pair object. Relax
the behavior of mbedtls_ecp_keypair so that you can extract just the parts
that you need.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/suites/test_suite_ecp.function b/tests/suites/test_suite_ecp.function
index 58d54ed..a4c86e2 100644
--- a/tests/suites/test_suite_ecp.function
+++ b/tests/suites/test_suite_ecp.function
@@ -1239,6 +1239,20 @@
         /* Check consistency with the group id */
         TEST_EQUAL(export_grp.id,
                    mbedtls_ecp_keypair_get_group_id(&key));
+
+        /* Test null arguments */
+        mbedtls_ecp_group_free(&export_grp);
+        mbedtls_mpi_free(&export_d);
+        mbedtls_ecp_point_free(&export_Q);
+        mbedtls_ecp_group_init(&export_grp);
+        mbedtls_mpi_init(&export_d);
+        mbedtls_ecp_point_init(&export_Q);
+        TEST_EQUAL(mbedtls_ecp_export(&key, &export_grp, NULL, NULL), 0);
+        TEST_EQUAL(mbedtls_ecp_group_cmp(&key.grp, &export_grp), 0);
+        TEST_EQUAL(mbedtls_ecp_export(&key, NULL, &export_d, NULL), 0);
+        TEST_EQUAL(mbedtls_mpi_cmp_mpi(&key.d, &export_d), 0);
+        TEST_EQUAL(mbedtls_ecp_export(&key, NULL, NULL, &export_Q), 0);
+        TEST_EQUAL(mbedtls_ecp_point_cmp(&key.Q, &export_Q), 0);
     }
 
 exit: