lanplus: Realloc the msg if the payload_length gets updated

It's possible the payload_length gets updated in
lanplus_encrypt_payload. If it's updated, the memory of msg should be
updated.

Tested: use ipmitool with lanplus with similar STR  and there is no
memory stomping issue.

Resolved: ipmitool/ipmitool#351
Signed-off-by: Tom Tung <shes050117@gmail.com>