Upon further investigation, I believe this crash is actually due to #1009325 in the radeon driver.
At least, after updating the radeon driver and xserver-xorg-core to sid latest, this doesn't occur anymore.
- Bug#1012026: X segfaults in OsLookupColor+0x135 after upg... Iustin Pop
- Bug#1012026: Confirming #1012026 on Debian sid brian m. carlson
- Bug#1012026: I believe this was actually the radeon ... Iustin Pop
