vnc_write() should be locked after the RFB protocol is initialized.

Fixes: 0c426e45 ("vnc: Add support for color map")
Cc: qemu-stable@nongnu.org
Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Message-id: 20201116141338.148911-1-liangpeng10@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>