bpffeature: EINVAL with empty logbuf should be false
bpffeature: EINVAL with empty logbuf should be false

On centos 7 `--info` currently prints all features as enabled. This is
due to the kernel not being able to handle `version=0`. As a result it
returns EINVAL with an empty logbuf. `strstr` on an empty string returns
no matches and the result will be true, which is obviously wrong.

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