[LTP] ltp_tpci.c: fix a null pointer

[Yang Xu]

Hi Po-Hsu

> We have encountered this null pointer dereference issue too, with this
> fix the test can be executed properly.
Thanks for your test.

Best Regards
Yang Xu
> 
> Tested-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
> 
>