Fixed problem found by fuzzing when Encrypt pointing to invalid reference

pdf/core/fuzz_test.go

@@ -104,3 +104,22 @@ endstream
 		t.Errorf("Should fail with an error")
 	}
 }
+
+// Problem where:
+//
+func TestFuzzIsEncryptedFail1(t *testing.T) {
+	parser := PdfParser{}
+	parser.rs, parser.reader = makeReaderForText(" /Name")
+
+	ref := &PdfObjectReference{ObjectNumber: -1}
+
+	parser.trailer = MakeDict()
+	parser.trailer.Set("Encrypt", ref)
+
+	_, err := parser.IsEncrypted()
+	if err == nil {
+		t.Errorf("err == nil: %v.  Should fail.", err)
+		return
+	}
+
+}

pdf/core/parser.go

@@ -1487,7 +1487,14 @@ func (this *PdfParser) IsEncrypted() (bool, error) {
 			if err != nil {
 				return false, err
 			}
-			encDict, ok := encObj.(*PdfIndirectObject).PdfObject.(*PdfObjectDictionary)
+
+			encIndObj, ok := encObj.(*PdfIndirectObject)
+			if !ok {
+				common.Log.Debug("Encryption object not an indirect object")
+				return false, errors.New("Type check error")
+			}
+			encDict, ok := encIndObj.PdfObject.(*PdfObjectDictionary)
+
 			common.Log.Trace("2: %q", encDict)
 			if !ok {
 				return false, errors.New("Trailer Encrypt object non dictionary")