Adding the author to the CC list.

Vedran thanks for bisecting this :-)
I'm not sure how this patch can cause such an issue. Either something funny has
happened during bisect or there is another underlying problem.

Summary of the problem so far:

Optimus system running Intel mesa and the proprietary Nvidia drivers. Running
Steam (games) with either bumblebee, optirun or primusrun seems to cause a
crash.

Steam Log:
Game update: AppID 550 "Left 4 Dead 2", ProcID 9399, IP 0.0.0.0:0
...
malloc: unknown:0: assertion botched
free: called with unallocated block argument
last command: (null)
Aborting...Game removed: AppID 550 "Left 4 Dead 2", ProcID 9399

Bisecting libdrm shows ae8edc7544e as the offending commit.

If an Intel dev has a similar machine they can try to reproduce it locally.
Otherwise a back-trace should help at least a bit.

I while back I was able to use gdb with steam. Not sure if it still works
* Close all steam games and the steam client
* $ export GAME_DEBUGGER=gdb
* $ steam

Note I'm not looking into the issue, just trying to get some information for
the Intel devs.