add NULL check for exec
add NULL check for exec

Signed-off-by: HaoZhang <30005fb020d50e8efc10620c1442bb709ef16c32@zju.edu.cn>

gnawux/hyperstart,hyperhq/hyperstart,gao-feng/hyperstart,gnawux/hyperstart,gnawux/hyperstart,laijs/hyperstart,gao-feng/hyperstart,laijs/hyperstart,hyperhq/hyperstart,bergwolf/hyperstart,bergwolf/hyperstart