Fix possible issue with NPE in PdfDictionaryEntrySet and PdfDictionaryValues.

DEVSIX-828

Author: pavel-alay

kernel/src/main/java/com/itextpdf/kernel/pdf/PdfDictionaryEntrySet.java

@@ -120,7 +120,7 @@ public PdfName getKey() {
         @Override
         public PdfObject getValue() {
             PdfObject obj = entry.getValue();
-            if (obj.isIndirectReference()) {
+            if (obj != null && obj.isIndirectReference()) {
                 obj = ((PdfIndirectReference) obj).getRefersTo(true);
             }
             return obj;

kernel/src/main/java/com/itextpdf/kernel/pdf/PdfDictionaryValues.java

@@ -119,7 +119,7 @@ public boolean hasNext() {
         @Override
         public PdfObject next() {
             PdfObject obj = parentIterator.next();
-            if (obj.isIndirectReference()) {
+            if (obj != null && obj.isIndirectReference()) {
                 obj = ((PdfIndirectReference) obj).getRefersTo(true);
             }
             return obj;