In case the string doesn't fit into the buffer snprintf returns the size
it would need, so len can be larger than the buffer.  Fix this by simply
using g_strdup_printf() instead of a static buffer.

Reported-by: Wenxiang Qian <leonwxqian@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20200701181801.27935-1-kraxel@redhat.com