vnc: Set invalid buffer pointers to NULL

After qemu_free, the pointers for input and output
buffers are no longer valid, so set them to NULL
(most other calls of qemu_free in vnc.c use this
pattern, too).

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
