PIVOT-954: Resolve "IndexOutOfBoundsException" thrown from TextAreaSkin
when you have either an empty document or any document where the first
line has nothing (except the line ending), and you double click on the
first line (or any line in the case of an empty document).

Basically we just need to catch the "start" value being negative before
trying to get the character there.


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1611829 13f79535-47bb-0310-9956-ffa450edef68