[HeapViewer] NPE from processGCRootReference()

[thurka]

Steps to reproduce:

1. open heap dump in VisualVM 2.1.7
2. select instance
3. display path to GC
4. NPE, see below

java.lang.NullPointerException
    at org.graalvm.visualvm.heapviewer.java.impl.HeapPatterns.processGCRootReference(HeapPatterns.java:48)
    at org.graalvm.visualvm.heapviewer.java.impl.PathToGCRootPlugin.computeInstanceRoots(PathToGCRootPlugin.java:384)
    at org.graalvm.visualvm.heapviewer.java.impl.PathToGCRootPlugin.access$500(PathToGCRootPlugin.java:111)
    at org.graalvm.visualvm.heapviewer.java.impl.PathToGCRootPlugin$1.computeData(PathToGCRootPlugin.java:191)
    at org.graalvm.visualvm.heapviewer.ui.TreeTableView$1.lazilyComputeChildren(TreeTableView.java:148)
    at org.graalvm.visualvm.heapviewer.model.HeapViewerNode$1.doInBackground(HeapViewerNode.java:180)
    at org.graalvm.visualvm.heapviewer.model.HeapViewerNode$1.doInBackground(HeapViewerNode.java:166)
    at java.desktop/javax.swing.SwingWorker$1.call(SwingWorker.java:304)
    at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    at java.desktop/javax.swing.SwingWorker.run(SwingWorker.java:343)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:834)

[thurka]

Fixed in master.