Correct completely broken os.stat behavior on Windows XP.

After 1a3e8db28d49, Windows XP could not os.stat at all due to raising
immediately when GetFinalPathNameByHandle wasn't available (pre-Vista).
The proper behavior in that situation is to just not attempt a traversal
rather than outright rejecting.

This change additionally handles a failed malloc by setting the error code
and returning false.

Patch by Hirokazu Yamamoto.