net: llmnr_responder: Check return value of net_udp_get_hdr

net_udp_get_hdr() function returns NULL on failure. Therefore
handle its return value to avoid potential NULL dereference.

Fixes #11485
Coverity-CID: 189738

Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
1 file changed