libxl: fix memory leak in libxl__strndup()

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_internal.c

@@ -155,6 +155,8 @@ char *libxl__strndup(libxl__gc *gc, const char *c, size_t n)
 
     if (!s) libxl__alloc_failed(CTX, __func__, n, 1);
 
+    libxl__ptr_add(gc, s);
+
     return s;
 }