[9] Review request for 8151015: JTextArea.insert() does not behave as expected with invalid position

[Semyon Sadetsky]

Hello,

Please review fix for JDK9:
bug: https://bugs.openjdk.java.net/browse/JDK-8151015
webrev: http://cr.openjdk.java.net/~ssadetsky/8151015/webrev.00/

It is regression of 4496801. The fix for 4496801 was wrong and the 
correct fix should be taking into account the implied character at the 
end of the document. Reverting 4496801 fixes 8151015. Also the correct 
fix for 4496801 is provided.

--Semyon