6716785: implicit null checks not triggering with CompressedOops

Allocate alignment-sized page(s) below java heap so that memory accesses at heap_base+1page give signal and cause an implicit null check

Reviewed-by: kvn, jmasa, phh, jcoomes

hotspot/src/share/vm/gc_implementation/parallelScavenge/parMarkBitMap.cpp

@@ -61,6 +61,8 @@ ParMarkBitMap::initialize(MemRegion covered_region)
   if (_virtual_space != NULL) {
     delete _virtual_space;
     _virtual_space = NULL;
+    // Release memory reserved in the space.
+    rs.release();
   }
   return false;
 }