net/lwip: fix the coding error
net/lwip: fix the coding error

This patch prevent approaching lladdr opt pointer after
it became null pointer

Change-Id: I6a623b01a8322f6bc7aae64c68e6e6d35061de36
Signed-off-by: jisuu.kim <800dec47d2987cee7cc3d7aee71bc11b56f5042e@samsung.com>

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