Thai character causes display failure on Mac

Build the attached main.cpp against Qt 4.6

It contains a UTF-8 string from http://www.columbia.edu/kermit/utf8.html :

ฉันกินกระจกได้ แต่มันไม่ทำให้ฉันเจ็บ

The 20th character - ำ - causes an assert in debug mode and a failure to render.

ASSERT: "num_glyphs <= length" in file /Users/sarasmit/depot/qt/qt/src/gui/text/qtextengine_mac.cpp, line 53

Note that this character corresponds to two seperate glyphs - glyphID's 103 and 78 - hence the "num_glyphs" is one greater than the "length".

The attached image shows this character being rendered by the QtCreator editor (highlighted in blue, showing the two seperate sections). Note that removing this character from the string will cause the complete Thai string to render, with no assert.

This bug was discovered while resolving QTBUG-4442