[NETFILTER]: nf_conntrack_h323: check range first in sequence extension

Check range before checking STOP flag. This optimization may save a
nanosecond or less :)

Signed-off-by: Jing Min Zhao <zhaojingmin@vivecode.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>