Following assertion failed on 2019-08-07 nightly kitchensink-jcstress test. The cause is actually due to the bad assertion itself, as _index == _size is possible when array is full. Ironically, the same assertions are handled correctly elsewhere. 

        #  Internal Error (/home/jenkins/workspace/nightly/shenandoah-jdk/src/hotspot/share/gc/shenandoah/shenandoahNMethod.cpp:354), pid=123793, tid=123808
        #  assert(_index >= 0 && _index < _size) failed: Sanity
        #
        # JRE version: OpenJDK Runtime Environment (14.0) (fastdebug build 14-internal+0-adhoc.jenkins.shenandoah-jdk)
        # Java VM: OpenJDK 64-Bit Server VM (fastdebug 14-internal+0-adhoc.jenkins.shenandoah-jdk, mixed mode, sharing, tiered, compressed oops, shenandoah gc, linux-amd64)
        # Problematic frame:
        # V  [libjvm.so+0x16565e4]  ShenandoahNMethodTable::remove(int)+0x164
        #
        # No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
        #