xwininfo segfaults if given a non-existent screen with -display option

[Impact]
xwininfo segfaults if called with an out-of-range display number.

[Test Case]
1. xwininfo -root -display :0.0
2. xwininfo -root -display :0.1

Actual Behavior:
 * First command passes
 * Second command segfaults
Expected Behavior:
 * First command passes
 * Second command returns an error message without segfaulting

[Fix]
The proposed patch works as is and is proposed for both precise and quantal.

[Regression Potential]
The patch is small and only affects an error condition check, so the chance of a regression seems very small. xwininfo is not widely used, so the impact of a regression would be minimal as well.

[Original Report]
I have a dual-head setup that only presents one "screen" to X (subdivided by xinerama).

If I run xwininfo -root -display :0.0 it works as expected, giving the root window information for my single "screen".
If I run xwininfo -root -display :0.1 it segfaults.
I expect it to exit gracefully with an error message. On Ubuntu 8.04 this error message used to be "cannot open display :0.1".

The problem could lie in libxcb (I'm no expert) xcb_connection_has_error() no longer returning true in the -display :0.1 case, but I've attached a patch for xwininfo anyway.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: x11-utils 7.6+4
ProcVersionSignature: Ubuntu 3.2.0-27.43-generic 3.2.21
Uname: Linux 3.2.0-27-generic x86_64
NonfreeKernelModules: nvidia
.proc.driver.nvidia.gpus.0: Error: [Errno 21] Is a directory: '/proc/driver/nvidia/gpus/0'
.proc.driver.nvidia.registry: Binary: ""
.proc.driver.nvidia.version:
 NVRM version: NVIDIA UNIX x86_64 Kernel Module 295.49 Mon Apr 30 23:46:33 PDT 2012
 GCC version: gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)
.tmp.unity.support.test.0:

ApportVersion: 2.0.1-0ubuntu11
Architecture: amd64
CompizPlugins: [core,composite,opengl,compiztoolbox,decor,vpswitch,snap,mousepoll,resize,place,move,wall,grid,regex,imgpng,session,gnomecompat,animation,fade,unitymtgrabhandles,workarounds,scale,expo,ezoom,unityshell]
CompositorRunning: None
Date: Tue Jul 24 16:37:34 2012
DistUpgraded: Fresh install
DistroCodename: precise
DistroVariant: ubuntu
DkmsStatus:
 nvidia-current, 295.40, 3.2.0-23-generic, x86_64: installed
 nvidia-current, 295.40, 3.2.0-26-generic, x86_64: installed
 nvidia-current, 295.40, 3.2.0-27-generic, x86_64: installed
 nvidia-current-updates, 295.49, 3.2.0-27-generic, x86_64: installed
GraphicsCard:
 NVIDIA Corporation G98 [Quadro NVS 295] [10de:06fd] (rev a1) (prog-if 00 [VGA controller])
   Subsystem: NVIDIA Corporation Device [10de:062e]
InstallationMedia: Ubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120425)
JockeyStatus:
 xorg:nvidia_current - NVIDIA accelerated graphics driver (Proprietary, Disabled, Not in use)
 xorg:nvidia_current_updates - NVIDIA accelerated graphics driver (post-release updates) (Proprietary, Enabled, In use)
MachineType: Dell Inc. Precision WorkStation T3500
ProcEnviron:
 LANGUAGE=en_AU:en
 TERM=xterm
 PATH=(custom, user)
 LANG=en_AU.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.2.0-27-generic root=UUID=2769af30-df94-44f1-9846-7241763e2193 ro quiet splash vt.handoff=7
SourcePackage: x11-utils
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/12/2010
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A07
dmi.board.name: 09KPNV
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 7
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA07:bd04/12/2010:svnDellInc.:pnPrecisionWorkStationT3500:pvr:rvnDellInc.:rn09KPNV:rvrA00:cvnDellInc.:ct7:cvr:
dmi.product.name: Precision WorkStation T3500
dmi.sys.vendor: Dell Inc.
version.compiz: compiz 1:0.9.7.8-0ubuntu1.2
version.ia32-libs: ia32-libs 20090808ubuntu36
version.libdrm2: libdrm2 2.4.32-1ubuntu1
version.libgl1-mesa-dri: libgl1-mesa-dri 8.0.2-0ubuntu3.1
version.libgl1-mesa-dri-experimental: libgl1-mesa-dri-experimental N/A
version.libgl1-mesa-glx: libgl1-mesa-glx 8.0.2-0ubuntu3.1
version.nvidia-graphics-drivers: nvidia-graphics-drivers N/A
version.xserver-xorg-core: xserver-xorg-core 2:1.11.4-0ubuntu10.6
version.xserver-xorg-input-evdev: xserver-xorg-input-evdev 1:2.7.0-0ubuntu1.2
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:6.14.99~git20111219.aacbd629-0ubuntu2
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.17.0-1ubuntu4
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:0.0.16+git20111201+b5534a1-1build2