ImageConverter: only call setSeries on non-negative values

This was introduced in 370dfdf2b4b6e072ff3a20264bbcb8a722ec8c37.
Without this change, bfconvert will throw an IllegalArgumentException
unless the '-series' flag is used.
Merge pull request #740 from melissalinkert/bfconvert-fix

ImageConverter: only call setSeries on non-negative values