PDFBOX-5931: avoid NPE

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1922699 13f79535-47bb-0310-9956-ffa450edef68

Author: THausherr

pdfbox/src/main/java/org/apache/pdfbox/cos/COSArray.java

@@ -575,8 +575,12 @@ public int indexOfObject(COSBase object)
                     return i;
                 }
             }
-            else if (item.equals(object)
-                    || item instanceof COSObject && ((COSObject) item).getObject().equals(object))
+            else if (item.equals(object))
+            {
+                return i;
+            }
+            else if (item instanceof COSObject && ((COSObject) item).getObject() != null &&
+                      ((COSObject) item).getObject().equals(object))
             {
                 return i;
             }