Actually, looking at the sample code and the string_expandtabs()
implementation it's clear what happened: the test for overflow on line
3318 or 3331 or 3339 must have been optimized out by GCC.

This is very inconvenient because lots of buffer overflow protection
uses similar code; this means that code that has been audited and fixed
in the past will again be vulnerable after compilation by GCC 4.3.

I'm going to ask Martin von Loewis to give an opinion on this.

Thanks for bringing this up!