win: refactor uv_cpu_info

Fixes a couple of error handling issues:

* Don't free an uninitialized pointer when allocating memory for
  `cpu_infos` fails.
* Don't return a bogus error value when NtQuerySystemInformation fails.
  That function returns and NTSTATUS code instead of setting the last
  error.
* Don't clobber out parameters when an error happens.