in the attempt to fix 627393, changed IFX_FileRead's readBlock to return the length it reads

testing/libfuzzer/xfa_codec_fuzzer.h

 // Copyright 2016 The PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TESTING_LIBFUZZER_XFA_CODEC_FUZZER_H_
 #define TESTING_LIBFUZZER_XFA_CODEC_FUZZER_H_
 
 #include <memory>
 
 #include "core/fxcodec/codec/ccodec_progressivedecoder.h"
@@ skipping 25 matching lines @@
                                   bitmap->GetHeight());
     while (status == FXCODEC_STATUS_DECODE_TOBECONTINUE)
       status = decoder->ContinueDecode();
 
     return 0;
   }
 
  private:
   class Reader : public IFX_SeekableReadStream {
    public:
-    Reader(const uint8_t* data, size_t size) : m_data(data), m_size(size) {}
+    Reader(const uint8_t* data, size_t size)
+        : m_data(data), m_size(size), m_nCurPos(0) {}
     ~Reader() {}
 
     void Release() override {}
 
-    bool ReadBlock(void* buffer, FX_FILESIZE offset, size_t size) override {
+    bool IsEOF() override { return m_nCurPos >= m_size; }
+
+    size_t ReadBlock(void* buffer, FX_FILESIZE offset, size_t size) override {
       if (offset < 0 || static_cast<size_t>(offset) >= m_size)
-        return false;
+        return 0;
       if (offset + size > m_size)
         size = m_size - offset;
       if (size == 0)
-        return false;
+        return 0;
 
       memcpy(buffer, m_data + offset, size);
-      return true;
+      m_nCurPos = offset + static_cast<FX_FILESIZE>(size);
+      return size;
     }
 
-    FX_FILESIZE GetSize() override { return static_cast<FX_FILESIZE>(m_size); }
+    FX_FILESIZE GetSize() override { return m_size; }
 
    private:
     const uint8_t* const m_data;
-    size_t m_size;
+    FX_FILESIZE m_nCurPos;
+    FX_FILESIZE m_size;
   };
 };
 
 #endif  // TESTING_LIBFUZZER_XFA_CODEC_FUZZER_H_