When looking for \0, use clen which may be optimized.

Also, return EINVAL when returned string is empty.

This makes it similar to how it is implemented in *bsd and solaris.

Change-Id: I3e37ed25f47110eafd12c80291f7746de9db7b23
Reviewed-on: https://go-review.googlesource.com/c/go/+/606902


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