net/lwip: fix static analysis issue in icmp6
net/lwip: fix static analysis issue in icmp6

It is to fix static analysis issue in icmp6.

Change-Id: Iff2403929addad6596688f884421182d59ae563c
Signed-off-by: Changsoon Kim <c1120524009612ae00648f6848418e342ca4bc0a@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT