fix leak in tests (#714)

coverity cid: 116836

Author: kevinAlbs

src/libmongoc/tests/test-mongoc-mongohouse.c

@@ -111,6 +111,7 @@ cmd_started_cb (const mongoc_apm_command_started_t *event)
    /* If the ns does not match, return without validating. */
    if (strcmp (test->cursor_ns, ns) != 0) {
       bson_mutex_unlock (&test->mutex);
+      bson_free (ns);
       return;
    }