I did some more debugging and it looks like memory corruption. I can reproduce
the problem with an apitrace dump but when using an --enable-debug build it
runs fine. It always segfaults in a call to glDrawRangeElements. I had the same
problem with Dungeon Defenders before. See comment 2 in bug 62967. It would
always segfault at some random glDrawRangeElements call unrelated to SB and
corrupt the stack. If I try to play back the sam3 trace with
LIBGL_ALWAYS_SOFTWARE=yes it will instead abort with the error "LLVM ERROR:
Program used external function '' which could not be resolved!" during a call
to glDrawRangeElements.

The full sam3 trace is 231M if you want it. I tried to trim it but the trimmed
version would result in an floating point exception in some other gl call.

I'm using latest git mesa but my setup is a bit odd. I run 64-bit gentoo but
the default 32-bit libs in gentoo are precompiled older versions. I work around
that problem by having a 32-bit gentoo chroot with git mesa, libdrm and other
libs games use. I run 32-bit apps like this
LIBGL_DRIVERS_PATH=/mnt/32bit/usr/lib/dri
LD_LIBRARY_PATH="/mnt/32bit/lib:/mnt/32bit/usr/lib" ./steam.sh