ipv6: move dereference after check in fl_free()

There is a dereference before checking for NULL bug here.  Generally
free() functions should accept NULL pointers.  For example, fl_create()
can pass a NULL pointer to fl_free() on the error path.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>