Fix memory leak if dhcp renew send fails (ARMmbed#1858)

If dhcp_service_send_req() returns failure during renew, previously allocated
payload was not freed. Added check for return value and free for payload if
request sending is failed.

source/6LoWPAN/Thread/thread_dhcpv6_client.c

@@ -284,7 +284,7 @@ void thread_dhcpv6_renew(protocol_interface_info_entry_t *interface, if_address_
     payload_len = libdhcpv6_address_request_message_len(srv_data_ptr->clientLinkIdType, srv_data_ptr->serverLinkType, 0);
     payload_ptr = ns_dyn_mem_temporary_alloc(payload_len);
     if (payload_ptr == NULL) {
-        addr->state_timer = 200;//Retry after? should there be maximum 20 second retry
+        addr->state_timer = 200; //Retry after 20 seconds
         tr_error("Out of memory");
         return ;
     }
@@ -308,6 +308,11 @@ void thread_dhcpv6_renew(protocol_interface_info_entry_t *interface, if_address_
     libdhcpv6_generic_nontemporal_address_message_write(payload_ptr, &packetReq, &nonTemporalAddress, &serverLink);
     // send solicit
     srv_data_ptr->transActionId = dhcp_service_send_req(dhcp_client.service_instance, 0, srv_data_ptr, srv_data_ptr->server_address, payload_ptr, payload_len, dhcp_solicit_resp_cb);
+    if (srv_data_ptr->transActionId == 0) {
+        ns_dyn_mem_free(payload_ptr);
+        addr->state_timer = 200; //Retry after 20 seconds
+        tr_error("DHCP renew send failed");
+    }
 }
 
 void thread_dhcpv6_client_set_address(int8_t interface_id, dhcpv6_client_server_data_t *srv_data_ptr)