ipmi: Fix kernel panic at ipmi_ssif_thread()

msg_written_handler() may set ssif_info->multi_data to NULL
when using ipmitool to write fru.

Before setting ssif_info->multi_data to NULL, add new local
pointer "data_to_send" and store correct i2c data pointer to
it to fix NULL pointer kernel panic and incorrect ssif_info->multi_pos.

Signed-off-by: Joeseph Chang <joechang@codeaurora.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: stable@vger.kernel.org # 3.19-