[TUI] correctly display windows source files

[Eli Zaretskii]

> Date: Tue, 10 Oct 2006 15:23:05 +0200
> From: Denis PILAT <denis.pilat@st.com>
> Cc: gdb-patches@sourceware.org
> 
> About your remark,
> I've tried to ungetc(EOF) and I did not get any problem, after 
> ungetc(EOF) the next call to fgetc is still EOF.

That could be so, but I'd prefer that our code didn't do anything that
could fail, even theoretically.  We shouldn't call functions with
arguments that invoke undefined or uncertain behavior.

It should be a simple matter of adding a test for EOF.