fixed warning in case of empty filter
fixed warning in case of empty filter

opencTM/PF_RING,opencTM/PF_RING,ntop/PF_RING,ntop/PF_RING,opencTM/PF_RING,ntop/PF_RING,opencTM/PF_RING,ntop/PF_RING,ntop/PF_RING,ntop/PF_RING,ntop/PF_RING