Fix warning when compiling with -O3

/home/jmayer/work/wireshark/git/epan/dissectors/packet-lmp.c: In function ‘dissect_lmp’:
/home/jmayer/work/wireshark/git/epan/dissectors/packet-lmp.c:776:13: error: array subscript is below array bounds [-Werror=array-bounds]
        ti = proto_tree_add_item(lmp_tree,
             ^

Change-Id: Ie949b4280f71a8f9f480a8e1d6592098ee7e67b5
Reviewed-on: https://code.wireshark.org/review/3573
Reviewed-by: Jörg Mayer <jmayer@loplof.de>