netdev: Fix crash when interface option is changed to invalid value.

When trying to modify an interface option (e.g. remote IP of a GRE port) to
an invalid value, the vswitchd does crash. For instance:
 ovs-vsctl add-br br0
 ovs-vsctl add-port br0 gre0 -- set interface gre0 type=gre \
           options:remote_ip=10.0.0.2
 ovs-vsctl set interface gre0 options:remote_ip=9.9.9

The bug is caused by trying to dereference a NULL pointer. It was introduced
by the commit 9fff138. Before that, the NULL pointer was handled by the
VLOG_WARN_BUF macro.

Signed-off-by: Zoltán Balogh <[email protected]>
CC: Daniele Di Proietto <[email protected]>
Fixes: 9fff138 ("netdev: Add 'errp' to set_config().")
Signed-off-by: Ben Pfaff <[email protected]>

lib/netdev.c

@@ -469,7 +469,11 @@ netdev_set_config(struct netdev *netdev, const struct smap *args, char **errp)
                           "%s: could not set configuration (%s)",
                           netdev_get_name(netdev), ovs_strerror(error));
             if (verbose_error) {
-                *errp = verbose_error;
+                if (errp) {
+                    *errp = verbose_error;
+                } else {
+                    free(verbose_error);
+                }
             }
         }
         return error;