RFR(XS): 8198906: JDK-8196882 breaks VS2013 Win32 builds

[Kim Barrett]

Please review this change to fix a build failure on Win32 when using
VS2013 (and likely earlier).  In os::vsnprintf, cast the int result to
size_t for comparison with the buffer size, after having verified the
result is non-negative.

I'm not sure why this failure doesn't occur with VS2013 Win64 builds.

CR:
https://bugs.openjdk.java.net/browse/JDK-8198906

Webrev:
http://cr.openjdk.java.net/~kbarrett/8198906/open.00/

Testing:
VS2013 Win64 still builds.  I don't have access to Win32, but the
change is pretty simple.