hw/block/m25p80: fix resource leak

These two are spot by Coverity 1357232 and 1357233.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1467684998-12076-1-git-send-email-zhaoshenglong@huawei.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
