Provide means to reset handshake cert list

Extend mbedtls_ssl_set_hs_own_cert() to reset handshake cert list
if cert provided is null.  Previously, mbedtls_ssl_set_hs_own_cert()
only provided a way to append to the handshake certificate list,
without providing a way to replace the handshake certificate list.

Signed-off-by: Glenn Strauss <gstrauss@gluelogic.com>
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index 4f5ee97..0c8928f 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -854,6 +854,15 @@
     ret = mbedtls_ssl_conf_own_cert( &( ep->conf ), &( cert->cert ),
                                      &( cert->pkey ) );
     TEST_ASSERT( ret == 0 );
+    TEST_ASSERT( ep->conf.key_cert != NULL );
+
+    ret = mbedtls_ssl_conf_own_cert( &( ep->conf ), NULL, NULL );
+    TEST_ASSERT( ret == 0 );
+    TEST_ASSERT( ep->conf.key_cert == NULL );
+
+    ret = mbedtls_ssl_conf_own_cert( &( ep->conf ), &( cert->cert ),
+                                     &( cert->pkey ) );
+    TEST_ASSERT( ret == 0 );
 
 exit:
     if( ret != 0 )