Error on negative value of LISTEN_FDS

Hi! We use a static code analyser on our code, which also checks dependencies. The analyser pointed out that in the code `activation/files_unix.go` there are no checks if `nfds` is negative, which may lead to a panic on line 58. It seems to me that the check on line 52 should be changed to
```go
if err != nil || nfds <= 0 {
```