tools api io: Ensure line_len_out is always initialized

tools api io: Ensure line_len_out is always initialized

Ensure initialization to avoid compiler warnings about potential use
of uninitialized variables.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Yoshihiro Furudera <fj5100bi@fujitsu.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ze Gao <zegao2021@gmail.com>
Cc: Changbin Du <changbin.du@huawei.com>
Cc: Junhao He <hejunhao3@huawei.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Link: https://lore.kernel.org/r/20241109003759.473460-2-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>