runtime: handle decommit failure gracefully on Windows
runtime: handle decommit failure gracefully on Windows

I have no test case for this at tip.
The original report included a program crashing at revision 88ac7297d2fa.
I tested this code at that revision and it does fix the crash.
However, at tip the reported code no longer crashes, presumably
because some allocation patterns have changed. I believe the
bug is still present at tip and that this code still fixes it.

Fixes issue 7143.

LGTM=alex.brainman
R=golang-codereviews, alex.brainman
CC=dvyukov, golang-codereviews
https://codereview.appspot.com/96300046

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang