Preprocessor allocate buffer using mmap() or malloc() for reading include files
at cppfiles.c / read_include_file().
When using malloc(), the buffer can be NUL-terminated.
But using mmap(), the buffer cannot be NUL-terminated.

Almost implementation of mmap() are NUL-terminated after the mmap()ed file data.
But XFS(SGI's file system) on Linux is different.

So everywhere reading buffer like 'buffer->cur++', must check buffer overflow.
For example, at cpplex.c / _cpp_lex_direct(),
> skipped_white:
+  if( buffer->cur > buffer->rlimit )
+    c = '\0';
+  else
>  c = *buffer->cur++;
>  result->col = CPP_BUF_COLUMN (buffer, buffer->cur);