Fix crash with XV with large virtual display

If a virtual display with width > 2048 is used, the first time
an XV buffer is needed will result in a BadAlloc error message,
but the next time X would crash.

Signed-off-by: Eric Anholt <eric@anholt.net>

src/i830_video.c

@@ -2392,6 +2392,7 @@ I830PutImage(ScrnInfoPtr pScrn,
                 return BadAlloc;
             if (!pPriv->textured && drm_intel_bo_pin(pPriv->buf, 4096) != 0) {
                 drm_intel_bo_unreference(pPriv->buf);
+                pPriv->buf = NULL;
                 xf86DrvMsg(pScrn->scrnIndex, X_ERROR,
                            "Failed to pin xv buffer\n");
                 return BadAlloc;