[windows]: test the syscall return value instead of the error

LazyProc.Call always returns a non-nil error: syscall.Errno is 0 on
success but still a non-nil interface value. `err != nil` was therefore
constant in four conditions, which staticcheck 0.8.0 reports as SA4023.

Each had a second term doing the real work, so behaviour is unchanged
except in the buffer-sizing call in cpu_windows.go, where success used to
return "failed to get buffer size: The operation completed successfully".
A zero length is now rejected explicitly instead of reaching &buffer[0]
on an empty slice.