Make xf86CompatOutput() return NULL when there are no privates

Some drivers (mach64 w/o DRI for instance) don't initialize privates.
Signed-off-by: Matthieu Herrb <matthieu.herrb@laas.fr>

hw/xfree86/modes/xf86Crtc.h

@@ -839,6 +839,9 @@ xf86CompatOutput(ScrnInfoPtr pScrn)
 {
     xf86CrtcConfigPtr config = XF86_CRTC_CONFIG_PTR(pScrn);
 
+    if (xf86CrtcConfigPrivateIndex == -1)
+        return NULL;
+
     if (config->compat_output < 0)
         return NULL;
     return config->output[config->compat_output];