I'm pushing a slightly modified version of Roland's patch.

(In reply to Nicolai Hähnle from comment #96)
> Created attachment 120742 [details]
> more conservative apitrace patch
> 
> For what it's worth, I've attached a modified version of Roland's patch that
> is slightly more conservative, guarding against some stupid end values and
> checking the indices. Not sure which patch is really better though, in the
> end it depends on how much broken software is out there. As far as I can
> tell, XCOM apitraces work with both variants.

Yes, I don't think this is necessary.  If apitrace needs more resiliency, then
the best approach would be to setup a segv handler to cope with out-of-bounds
reads.

This code path is only used for user arrays. (VBOs don't need this special
treatment.)