net/lwip: clean up the validation of NS message
net/lwip: clean up the validation of NS message

This patch clean up the validation of NS message and
also add one more exception handling for NS message.
There's no multicast address possible at target address
field in NS message.

Change-Id: Ia567d80bcd2b0380fdb86dd982080f617b7f4a31
Signed-off-by: jisuu.kim <800dec47d2987cee7cc3d7aee71bc11b56f5042e@samsung.com>

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