Check if offset is valid before using it

[FlorianKroiss]

The offset might be invalid when a user clicks on a location which is after the last character of a line.
The existing check no longer works, because getOffsetAtPoint only throws an IllegalArgumentException when the given Point is null.

[fabioz]

Maybe it could keep catching the exception (so that it keeps working the same on some older version of Eclipse -- i.e.: not sure when it stopped raising exception and started returning -1).

[fabioz]

p.s.: Thank you for the PR :)

[FlorianKroiss]

The method did not change it's behavior. I rather think that this a regression from ba5f4f0, where the deprecated getOffsetAtLocation method (which threw the exception) was replaced with the recommended getOffsetAtPoint method which returns -1 in case of an invalid offset. So I think the new handling code should be fine and we don't need to keep the try-catch

[fabioz]

Great, thank you for taking the time to digg this up. Just merged.