Fix memory leak when advertising route (ARMmbed#1814)

Leak of 40 bytes occurs every time ipv6_stack_route_advert_update is
called. Do new prefix allocation only if prefix is not found from
interface address list and prefix is not already allocated.

source/ipv6_stack/protocol_ipv6.c

@@ -394,14 +394,14 @@ void ipv6_stack_route_advert_update(uint8_t *address, uint8_t prefixLength, uint
  return;
  }
 
+ if (addr_interface_gp_prefix_compare(cur, address) == 0) {
+ return;
+ }
 
-
- if (addr_interface_gp_prefix_compare(cur, address) != 0) {
- ns_list_foreach(ipv6_interface_route_on_link_t, cur_prefix, &route_on_link) {
- if ((cur_prefix->prefix_len == prefixLength) && bitsequal(cur_prefix->prefix, address, prefixLength)) {
- cur_prefix->routePrefer = routePrefer;
- return;
- }
+ ns_list_foreach(ipv6_interface_route_on_link_t, cur_prefix, &route_on_link) {
+ if ((cur_prefix->prefix_len == prefixLength) && bitsequal(cur_prefix->prefix, address, prefixLength)) {
+ cur_prefix->routePrefer = routePrefer;
+ return;
  }
  }