Comment 1 for bug 1056381

Here is the gdb output from apport-retrace:

GNU gdb (GDB) 7.5-ubuntu
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Reading symbols from /tmp/tmp8_Sp88/usr/bin/Xorg...Reading symbols from /tmp/tmp8_Sp88/usr/lib/debug/usr/bin/Xorg...done.
done.
[New LWP 901]
warning: Can't read pathname for load map: Input/output error.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
warning: the debug information found in "/tmp/tmp8_Sp88/usr/lib/debug/usr/lib/libXfont.so.1.4.1" does not match "/tmp/tmp8_Sp88/usr/lib/libXfont.so.1" (CRC mismatch).

Core was generated by `/usr/bin/X :0 -core -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitc'.
Program terminated with signal 6, Aborted.
#0 0x00007f6eaa5e2405 in __GI_raise (sig=<optimized out>)
    at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
64 ../nptl/sysdeps/unix/sysv/linux/raise.c: No such file or directory.
(gdb) where
#0 0x00007f6eaa5e2405 in __GI_raise (sig=<optimized out>)
    at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1 0x00007f6eaa5e5b6b in __GI_abort () at abort.c:91
#2 0x00007f6eaa5db0ce in __assert_fail_base (fmt=<optimized out>,
    assertion=0x7f6eac7da158 "!screen_specific_private[type]",
    file=0x7f6eac7da036 "../../dix/privates.c", line=<optimized out>,
    function=<optimized out>) at assert.c:94
#3 0x00007f6eaa5db172 in __GI___assert_fail (
    assertion=assertion@entry=0x7f6eac7da158 "!screen_specific_private[type]",
    file=file@entry=0x7f6eac7da036 "../../dix/privates.c", line=line@entry=560,
    function=function@entry=0x7f6eac7da490 <__PRETTY_FUNCTION__.13574> "dixPrivatesSize")
    at assert.c:103
#4 0x00007f6eac6924ae in dixPrivatesSize (type=type@entry=PRIVATE_PIXMAP)
    at ../../dix/privates.c:560
#5 0x00007f6ea94370ea in qxl_screen_init (pScreen=pScreen@entry=0x7f6eae798020,
    argc=argc@entry=11, argv=argv@entry=0x7fff504c7968) at ../../src/qxl_driver.c:1200
#6 0x00007f6eac675ce5 in AddScreen (pfnInit=0x7f6ea9436dd0 <qxl_screen_init>,
    argc=argc@entry=11, argv=argv@entry=0x7fff504c7968) at ../../dix/dispatch.c:3830
#7 0x00007f6eac6b73e3 in InitOutput (pScreenInfo=0x7f6eac6affd0 <xf86SetDGAMode>,
    pScreenInfo@entry=0x7f6eaca56da0 <screenInfo>, argc=argc@entry=11,
    argv=argv@entry=0x7fff504c7968) at ../../../../hw/xfree86/common/xf86Init.c:913
#8 0x00007f6eac664386 in main (argc=11, argv=0x7fff504c7968, envp=<optimized out>)
    at ../../dix/main.c:204