libxl: arm: do not create /chosen/bootargs in DTB if no cmdline is specified

Otherwise we deference a NULL pointer.

I saw this while experimenting with libvirt on Xen on ARM, xl already checks
that the command line is non NULL and provides "" as a default.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Cc: george.dunlap@citrix.com>

tools/libxl/libxl_arm.c

@@ -164,8 +164,10 @@ static int make_chosen_node(libxl__gc *gc, void *fdt,
     res = fdt_begin_node(fdt, "chosen");
     if (res) return res;
 
-    res = fdt_property_string(fdt, "bootargs", info->u.pv.cmdline);
-    if (res) return res;
+    if (info->u.pv.cmdline) {
+        res = fdt_property_string(fdt, "bootargs", info->u.pv.cmdline);
+        if (res) return res;
+    }
 
     res = fdt_end_node(fdt);
     if (res) return res;