RFR (XS) 8229000: Fix lock and reenable assert in Monitor::check_safepoint_state

[Kim Barrett]

> On Aug 2, 2019, at 8:37 AM, coleen.phillimore at oracle.com wrote:
> 
> There was only one place in CMS that violated the condition in the new assert, fixed with this patch.  See linked bugs for details.
> 
> open webrev at http://cr.openjdk.java.net/~coleenp/2019/8229000.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8229000
> 
> Ran mach5 hs-tier1-3 (all failed tests were in these tiers) on linux-x64-debug, and tier1 on all Oracle platforms.
> 
> Thanks,
> Coleen

Looks good.