netfilter: nf_nat_core: merge the same lines

proto->unique_tuple() will be called finally, if the previous calls fail. This
patch checks the false condition of (range->flags &IP_NAT_RANGE_PROTO_RANDOM)
instead to avoid duplicate line of code: proto->unique_tuple().

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>