libxl: fix double free of ifname, when makes args for qemu.

In libxl_build_device_model_args_new, vifs[i].ifname can be free two
times, by the gc, and by freeing the vifs structures. This patch avoids
this.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
committer: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
---
 tools/libxl/libxl.c |   10 +++++++---
 1 files changed, 7 insertions(+), 3 deletions(-)

tools/libxl/libxl.c

@@ -1341,14 +1341,18 @@ static char ** libxl_build_device_model_args_new(libxl__gc *gc,
                 char *smac = libxl__sprintf(gc, "%02x:%02x:%02x:%02x:%02x:%02x",
                                            vifs[i].mac[0], vifs[i].mac[1], vifs[i].mac[2],
                                            vifs[i].mac[3], vifs[i].mac[4], vifs[i].mac[5]);
-                if (!vifs[i].ifname)
-                    vifs[i].ifname = libxl__sprintf(gc, "tap%d.%d", info->domid, vifs[i].devid);
+                char *ifname;
+                if (!vifs[i].ifname) {
+                    ifname = libxl__sprintf(gc, "tap%d.%d", info->domid, vifs[i].devid);
+                } else {
+                    ifname = vifs[i].ifname;
+                }
                 flexarray_set(dm_args, num++, "-net");
                 flexarray_set(dm_args, num++, libxl__sprintf(gc, "nic,vlan=%d,macaddr=%s,model=%s",
                             vifs[i].devid, smac, vifs[i].model));
                 flexarray_set(dm_args, num++, "-net");
                 flexarray_set(dm_args, num++, libxl__sprintf(gc, "tap,vlan=%d,ifname=%s,script=no",
-                            vifs[i].devid, vifs[i].ifname));
+                            vifs[i].devid, ifname));
                 ioemu_vifs++;
             }
         }