[patch] fix regression preprocessor/17610

[Zack Weinberg]

neroden@fastmail.fm (Nathanael Nerode) writes:

> Solves the bug (whee).  Bootstrapped & regression tested on i686-pc-linux-gnu.
>
> A testcase is included (although it is imperfect, since in the failing case,
> it has to wait and time out because it doesn't deal with stdin smoothly).
> Testing of the testcase is in progress.  OK to commit?

OK provided you verify that this doesn't break -include "" which is
*supposed* to read stdin.

This should go onto 3.4 branch as well.

zw