Fix a case in bbuf copy constructore

src/cpp/util/byte_buffer.cc

@@ -85,7 +85,9 @@ ByteBuffer::ByteBuffer(const ByteBuffer& buf)
 
 ByteBuffer& ByteBuffer::operator=(const ByteBuffer& buf) {
   Clear();                                       // first remove existing data
-  buffer_ = grpc_byte_buffer_copy(buf.buffer_);  // then copy
+  if (buf.buffer_) {
+    buffer_ = grpc_byte_buffer_copy(buf.buffer_);  // then copy
+  }
   return *this;
 }