cmUVStreambuf: Initialize all members on construction

Avoid leaving any members uninitialized after construction even if they
are later initialized before use by methods.  This helps convince static
analysis tools that the members are not used uninitialized.

Source/cmUVStreambuf.h

@@ -68,10 +68,10 @@ protected:
 
 private:
   uv_stream_t* Stream = nullptr;
-  void* OldStreamData;
-  const std::size_t PutBack;
+  void* OldStreamData = nullptr;
+  const std::size_t PutBack = 0;
   std::vector<CharT> InputBuffer;
-  bool EndOfFile;
+  bool EndOfFile = false;
 
   void StreamReadStartStop();