fix exception if service_per_vm is requested to delete a non-existing node
fix exception if service_per_vm is requested to delete a non-existing node

This happened during the unit tests, in particular in join_leave_SUITE:add_3_rm_3_data when stop_ring started killing dht_node processes for which service_per_vm still has delete_node messages in its queue.
seyyed/scalaris,seyyed/scalaris,digshock/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,seyyed/scalaris,digshock/scalaris,digshock/scalaris,digshock/scalaris,tectronics/scalaris,digshock/scalaris,seyyed/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris,tectronics/scalaris,tectronics/scalaris,tectronics/scalaris,seyyed/scalaris,tectronics/scalaris