colo-compare: fix the dangerous assignment

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Zhang Chen <zhangckid@gmail.com>
Cc: Li Zhijian <lizhijian@cn.fujitsu.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Fixes: 8ec14402
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>