GH-487 release snapshot data when shapshot component is closed

Author: thurka

visualvm/profiling/src/org/graalvm/visualvm/profiling/snapshot/ProfilerSnapshotNPS.java

@@ -147,6 +147,7 @@ public void run() {
                                 "removed", invocationTargetException);   // NOI18N
                     }
                     srw = null;
+                    closeSnapshot();
                 }
             });
         }