don't leave ignore_error_nr[etype] with 4 on blank input or error

don't leave ignore_error_nr[etype] with 4 on blank input or error

This isn't a bug since td_non_fatal_error() checks errnos to ignore
by td->o.ignore_error[etype] being NULL or not, but it's still strange
to leave td->o.ignore_error_nr[etype] with 4 (or multiple of 4 on
error) when there aren't any.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>