libxl: Fix SEGV in network-attach

When "device/vif" directory exists but is empty l!=NULL, but nb==0, so
l[nb-1] is invalid.  Add missing check.

Signed-off-by: Marek Marczykowski <marmarek@invisiblethingslab.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
(cherry picked from commit 9f1a6ff38b)

Conflicts:
	tools/libxl/libxl.c
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl.c

@@ -1207,7 +1207,8 @@ int libxl_device_nic_add(libxl_ctx *ctx, uint32_t domid, libxl_device_nic *nic)
             goto out_free;
         }
         if (!(l = libxl__xs_directory(&gc, XBT_NULL,
-                                     libxl__sprintf(&gc, "%s/device/vif", dompath), &nb))) {
+                                     libxl__sprintf(gc, "%s/device/vif", dompath), &nb)) ||
+            nb == 0) {
             nic->devid = 0;
         } else {
             nic->devid = strtoul(l[nb - 1], NULL, 10) + 1;