Fix possible crash on exit due to wrong vector size (#201)
Fix possible crash on exit due to wrong vector size (#201)

On method BPFtrace::find_empty_key() size of 'value' vector (value_size)
must be multiplied by ncpus_ when map.type_.type == Type::lhist
otherwise bpf syscall (BPF_MAP_LOOKUP_ELEM) will write out of bounds
thus corrupting the heap.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace