hw/xen: Check if len is 0 before memcpy()

data->data can be NULL when len is 0. Strictly speaking, the behavior
of memcpy() in such a scenario is undefined so UBSan complaints.

Satisfy UBSan by checking if len is 0 before memcpy().

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>

hw/i386/kvm/xen_xenstore.c

@@ -532,6 +532,10 @@ static void xs_read(XenXenstoreState *s, unsigned int req_id,
         return;
     }
 
+    if (!len) {
+        return;
+    }
+
     memcpy(&rsp_data[rsp->len], data->data, len);
     rsp->len += len;
 }