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/psParallelCompact.cpp

@@ -422,6 +422,8 @@ ParallelCompactData::create_vspace(size_t count, size_t element_size)
       return vspace;
     }
     delete vspace;
+    // Release memory reserved in the space.
+    rs.release();
   }
 
   return 0;