stkutil: Make valgrind happy

stkutil: Make valgrind happy

==29809== Conditional jump or move depends on uninitialised value(s)
==29809==    at 0x4E826C: stk_file_iter_next (stkutil.c:212)
==29809==    by 0x4E8CF8: parse_dataobj_file_list (stkutil.c:635)
==29809==    by 0x4EBA29: parse_dataobj (stkutil.c:2410)
==29809==    by 0x4ECFB5: parse_refresh (stkutil.c:2971)
==29809==    by 0x4EECA3: parse_command_body (stkutil.c:3826)
==29809==    by 0x4EF0DF: stk_command_new_from_pdu (stkutil.c:3948)
==29809==    by 0x4D50DA: ofono_stk_proactive_command_handled_notify
(stk.c:2885)