Fix debug macro situation (#5114)

Use the standard NDEBUG.

src/ByteBuffer.cpp

@@ -115,7 +115,7 @@ bool cByteBuffer::Write(const void * a_Bytes, size_t a_Count)
 
 	// Store the current free space for a check after writing:
 	size_t CurFreeSpace = GetFreeSpace();
-	#ifdef _DEBUG
+	#ifndef NDEBUG
 		size_t CurReadableSpace = GetReadableSpace();
 	#endif
 	size_t WrittenBytes = 0;