Fix memory leak in TLS 1.3 record protection unit test

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index a83d6e2..554e7b8 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -4024,6 +4024,7 @@
     ASSERT_COMPARE( rec.buf + rec.data_offset, rec.data_len,
                     plaintext->x, plaintext->len );
 
+    mbedtls_free( buf );
     mbedtls_ssl_transform_free( &transform_send );
     mbedtls_ssl_transform_free( &transform_recv );
 }