convert_links backup-name rewrite can underflow stack allocation

In [REDACTED], the code that rewrites an existing .html backup name into .orig allocates filename_len + 1 bytes on the stack, copies the whole filename, then blindly overwrites four bytes starting at (filename_plus_orig_suffix + filename_len) - 4 with "orig". For short filenames this writes before the alloca buffer and can corrupt the stack.