Fix crash in runexecutor on systems without cpuacct cgroup because of missing cputime measurement.
Fix crash in runexecutor on systems without cpuacct cgroup
because of missing cputime measurement.

martin-neuhaeusser/benchexec,sosy-lab/benchexec,martin-neuhaeusser/benchexec,IljaZakharov/benchexec,dbeyer/benchexec,IljaZakharov/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,IljaZakharov/benchexec,sosy-lab/benchexec,martin-neuhaeusser/benchexec,IljaZakharov/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,martin-neuhaeusser/benchexec