XFA: Fix the Mac build after commit 375a86403b7f.

xfa/src/fgas/src/crt/fx_stream.cpp

Index: xfa/src/fgas/src/crt/fx_stream.cpp
diff --git a/xfa/src/fgas/src/crt/fx_stream.cpp b/xfa/src/fgas/src/crt/fx_stream.cpp
index fb8c3c7fba5c95ca30557782940f5b1b4a960529..96633516027209d0d0540344ca1b46eafd26a2e2 100644
--- a/xfa/src/fgas/src/crt/fx_stream.cpp
+++ b/xfa/src/fgas/src/crt/fx_stream.cpp
@@ -1323,8 +1323,9 @@ FX_BOOL CFX_BufferAccImp::ReadBlock(void* buffer,
     return FALSE;
   }
   const uint8_t* pBuffer = m_pBufferRead->GetBlockBuffer();
-  FX_FILESIZE dwOffset = offset - dwBlockOffset;
-  size_t dwCopySize = std::min(size, dwBlockSize - dwOffset);
+  const FX_FILESIZE dwOffset = offset - dwBlockOffset;
+  size_t dwCopySize =
+      std::min(size, static_cast<size_t>(dwBlockSize - dwOffset));
   FXSYS_memcpy(buffer, pBuffer + dwOffset, dwCopySize);
   offset = dwCopySize;
   size -= dwCopySize;