Fix selecting single character in middle of string

Fix selecting single character in middle of string

The fix for QTBUG-46829 revealed a bug in the code to handle
selecting part of ligatures. The ranges were assumed to be
[start, end], while they are in fact [start, end>. This would
cause the engine to assume the previous node overlapped completely
with the selected node and that the node had thus already been
added to the graph.

Due to the bug in QTBUG-46829, this accidentally worked before,
but when that bug was fixed, this bug appeared.

Change-Id: I517d260de9f58db4504dd4320b7113fbbe305a81
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>