merge branch 'pr-1928'
merge branch 'pr-1928'

  rootless: fix potential panic in shouldUseRootlessCgroupManager

LGTMs: 2665d0b6daa6ccd1337288f8885cbc52b5d5ee7e@cyphar
Closes #1928

rhatdan/runc,justincormack/runc,crosbymichael/runc,mYmNeo/runc,mheon/runc,rhatdan/runc,mikebrow/runc,dqminh/runc,dqminh/runc,opencontainers/runc,estesp/runc,mYmNeo/runc,crosbymichael/runc,opencontainers/runc,mheon/runc,crosbymichael/runc,xiaochenshen/runc,kolyshkin/runc,kolyshkin/runc,kolyshkin/runc,opencontainers/runc,rhatdan/runc,mheon/runc,adrianreber/runc,mikebrow/runc,mikebrow/runc,xiaochenshen/runc,rhatdan/runc,adrianreber/runc,estesp/runc,laijs/runc,xiaochenshen/runc,laijs/runc,justincormack/runc,estesp/runc,justincormack/runc,mYmNeo/runc,dqminh/runc,dqminh/runc,adrianreber/runc,kolyshkin/runc,laijs/runc,opencontainers/runc