Bug 66425: Avoid a NullPointerException found via oss-fuzz

We try to avoid throwing NullPointerException, but it was possible
to trigger one here with a specially crafted input-file

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=61372

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1911603 13f79535-47bb-0310-9956-ffa450edef68

poi-scratchpad/src/main/java/org/apache/poi/hdgf/extractor/VisioTextExtractor.java

@@ -38,7 +38,7 @@ import org.apache.poi.poifs.filesystem.POIFSFileSystem;
  *  can return the text for you (example: for use with Lucene).
  */
 public final class VisioTextExtractor implements POIOLE2TextExtractor {
-    private HDGFDiagram hdgf;
+    private final HDGFDiagram hdgf;
     private boolean doCloseFilesystem = true;
 
     public VisioTextExtractor(HDGFDiagram hdgf) {
@@ -72,10 +72,12 @@ public final class VisioTextExtractor implements POIOLE2TextExtractor {
     private void findText(Stream stream, List<String> text) {
         if(stream instanceof PointerContainingStream) {
             PointerContainingStream ps = (PointerContainingStream)stream;
-            for(final Stream substream : ps.getPointedToStreams()) {
+            if (ps.getPointedToStreams() != null) {
+                for (final Stream substream : ps.getPointedToStreams()) {
                     findText(substream, text);
                 }
             }
+        }
         if(stream instanceof ChunkStream) {
             ChunkStream cs = (ChunkStream)stream;
             for(final Chunk chunk : cs.getChunks()) {