qxl: dont update invalid area

This patch fixes the following error:

$ ~/usr/bin/qemu-system-x86_64 -enable-kvm -m 1024 -spice port=5900,disable-ticketing -vga qxl -cdrom ~/Images/linuxmint-13-mate-dvd-32bit.iso
(/home/mathslinux/usr/bin/qemu-system-x86_64:10068): SpiceWorker-CRITICAL **: red_worker.c:4599:red_update_area: condition `area->left >= 0 && area->top >= 0 && area->left < area->right && area->top < area->bottom' failed
Aborted

spice server terminates QEMU process if we pass invalid area to it,
so dont update those invalid areas.
Signed-off-by: NDunrong Huang <riegamaths@gmail.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/qxl.c

@@ -1470,6 +1470,13 @@ async_common:
             return;
         }
 
+        if (update.left < 0 || update.top < 0 || update.left >= update.right ||
+            update.top >= update.bottom) {
+            qxl_set_guest_bug(d, "QXL_IO_UPDATE_AREA: "
+                              "invalid area(%d,%d,%d,%d)\n", update.left,
+                              update.right, update.top, update.bottom);
+            break;
+        }
         if (async == QXL_ASYNC) {
             cookie = qxl_cookie_new(QXL_COOKIE_TYPE_IO,
                                     QXL_IO_UPDATE_AREA_ASYNC);