lbm: fix "member access within null pointer" warning

This patch avoids the following warning with `clang -fsanitize=undefined`:

    epan/dissectors/packet-lbtrm.c:1519:36: runtime error: member access within null pointer of type 'lbtrm_hdr_t'

Change-Id: I973caf92547f5d16c0de20908b2b3fbc09227df0
Reviewed-on: https://code.wireshark.org/review/3313
Reviewed-by: Evan Huus <eapache@gmail.com>