leaseshelper: fix another crash

We create a 'lease_new' when we are adding new lease entry, then later
in the code we add the 'lease_new' into a 'leases_array_new' which
leads into the crash because we double free the 'lease_new'.

To prevent the double free we set the 'lease_new' to NULL after
successful append into the 'leases_array_new'.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/network/leaseshelper.c

@@ -331,6 +331,7 @@ main(int argc, char **argv)
                            _("failed to create json"));
             goto cleanup;
         }
+        lease_new = NULL;
     }
 
     if (!(leases_str = virJSONValueToString(leases_array_new, true))) {