RFR(xs): [windows] small fixes to os::check_heap()

[Thomas Stüfe]

Hi all,

could you please sponsor and review these tiny changes to os::check_heap()
on windows:

webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8143233/webrev.00/webrev/
bug: https://bugs.openjdk.java.net/browse/JDK-8143233?filter=22798

Fixes two issues:
- GetProcessHeap() is not necessarily the CRT heap, so we need to use
_get_heap_handle(); instead
- We need to always unlock the heap before exiting with fatal() in case we
found an error.

Kind Regards, Thomas