8297309: Memory leak in ShenandoahFullGC

Reviewed-by: rkennke, shade

src/hotspot/share/gc/shenandoah/shenandoahFullGC.cpp

@@ -66,6 +66,10 @@ ShenandoahFullGC::ShenandoahFullGC() :
   _gc_timer(ShenandoahHeap::heap()->gc_timer()),
   _preserved_marks(new PreservedMarksSet(true)) {}
 
+ShenandoahFullGC::~ShenandoahFullGC() {
+  delete _preserved_marks;
+}
+
 bool ShenandoahFullGC::collect(GCCause::Cause cause) {
   vmop_entry_full(cause);
   // Always success

src/hotspot/share/gc/shenandoah/shenandoahFullGC.hpp

@@ -66,6 +66,7 @@ private:
 
 public:
   ShenandoahFullGC();
+  ~ShenandoahFullGC();
   bool collect(GCCause::Cause cause);
 
 private: