RFR (S) 8193053: jvm crash by G1CMBitMapClosure::do_addr

[coleen.phillimore at oracle.com]

Summary: We were adding an unloaded mirror to the SATB collection set in 
remove_handle.

open webrev at http://cr.openjdk.java.net/~coleenp/8193053.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8193053

This fix is somewhat minimal because it's late in the jdk 10 release 
cycle but improved function name is welcome.

Tested with failing jck tests for several hours (usual failure in first 
run).  Also ran mach5 tier1-5 (still in progress but almost done).

Thanks,
Coleen