Check extraErr before accessing extraUsage.Bytes (#2401)
Check extraErr before accessing extraUsage.Bytes (#2401)

* Check extraErr before accessing extraUsage.Bytes

Signed-off-by: Ted Yu <22f3e725b314afeca4d45e12683c560674029d21@gmail.com>
linux-on-ibm-z/cadvisor,linux-on-ibm-z/cadvisor,linux-on-ibm-z/cadvisor