xserver/hw/xfree86/modes compat_output == -1 bad dereference

http://lists.x.org/archives/xorg-devel/2013-March/035751.html
http://lists.x.org/archives/xorg-devel/2013-April/035933.html

[PATCH] xf86: return NULL for xf86CompatOutput if config->compat_output is -1

If there is no compat output, config->compat_output is -1 and xf86CompatOutput
reads before the beginning of the outputs array.

 Invalid read of size 8
    at 0x4DB892: xf86CompatOutput (xf86Crtc.h:734)
    by 0x4E285D: xf86OutputSetEDID (xf86Crtc.c:2986)
    by 0x862200F: intel_output_attach_edid (intel_display.c:895)
    by 0x86222A5: intel_output_get_modes (intel_display.c:955)
    by 0x4DF296: xf86ProbeOutputModes (xf86Crtc.c:1586)
    by 0x4E1349: xf86InitialConfiguration (xf86Crtc.c:2380)
    by 0x8623F72: intel_mode_pre_init (intel_display.c:1788)
    by 0x8626088: I830PreInit (intel_driver.c:625)
    by 0x49873B: InitOutput (xf86Init.c:606)
    by 0x4257B8: main (main.c:204)
  Address 0x7955d88 is 8 bytes before a block of size 16 alloc'd
    at 0x4C2C25E: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x4DCD54: xf86OutputCreate (xf86Crtc.c:621)
    by 0x862322F: intel_output_init (intel_display.c:1416)
    by 0x8623F22: intel_mode_pre_init (intel_display.c:1780)
    by 0x8626088: I830PreInit (intel_driver.c:625)
    by 0x49873B: InitOutput (xf86Init.c:606)
    by 0x4257B8: main (main.c:204)

Signed-off-by: Aaron Plattner <aplattner@nvidia.com>
---
 hw/xfree86/modes/xf86Crtc.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/hw/xfree86/modes/xf86Crtc.h b/hw/xfree86/modes/xf86Crtc.h
index 802303f..1ac8485 100644
--- a/hw/xfree86/modes/xf86Crtc.h
+++ b/hw/xfree86/modes/xf86Crtc.h
@@ -731,6 +731,8 @@ xf86CompatOutput(ScrnInfoPtr pScrn)
 {
     xf86CrtcConfigPtr config = XF86_CRTC_CONFIG_PTR(pScrn);
 
+    if (config->compat_output < 0)
+        return NULL;
     return config->output[config->compat_output];
 }
 
-- 
1.8.2