I've been staring at the code. I just realized: we really should call path_error() as soon as possible once we detect the error--as Christian's patch points out, close() will clear the error. So instead of playing footsie with assigning (further) to errno, I'm leaving the if (dirp == NULL) code where it is. It won't happen any sooner execution-wise; moving the close up only makes it slightly(?) easier to read, at the expense of duplicating some code.
Christian, the simplicity of your patch misses one minor point: when FDOPENDIR is not defined, fd is never used, and therefore emits a warning. I was trying to clean that up at the same time as fixing this bug.