Make sure the fuzzer read size does not go negative.

testing/libfuzzer/xfa_codec_fuzzer.h

Index: testing/libfuzzer/xfa_codec_fuzzer.h
diff --git a/testing/libfuzzer/xfa_codec_fuzzer.h b/testing/libfuzzer/xfa_codec_fuzzer.h
index 6a84ed85725939eb030880f2652dc739dcd085f2..13a467e1ef7165003651ce45e58a487ca2deb176 100644
--- a/testing/libfuzzer/xfa_codec_fuzzer.h
+++ b/testing/libfuzzer/xfa_codec_fuzzer.h
@@ -49,8 +49,13 @@ class XFACodecFuzzer {
     void Release() override {}
 
     FX_BOOL ReadBlock(void* buffer, FX_FILESIZE offset, size_t size) override {
+      if (offset < 0 || offset >= m_size)
+        return FALSE;
       if (offset + size > m_size)
         size = m_size - offset;
+      if (size == 0)
+        return FALSE;
+
       memcpy(buffer, m_data + offset, size);
       return TRUE;
     }