[bug 3255] comboboxex fix + tests

[Ann & Jason Edmeades]

The bug itself was an off by one calculation due to zero based indexes, but
I also saw the same problem existed in other functions and so wrote some
small tests for them. 

Changelog

Fix some off by one calculations in the comboboxex functions, and handle an
out of range positive index the same as windows + unit test case

Jason
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