Fix pressing down in the TUI (Re: [RFC 8.3 0/3] Some style fixes)

[Eli Zaretskii]

> Cc: tromey@adacore.com, gdb-patches@sourceware.org
> From: Pedro Alves <palves@redhat.com>
> Date: Fri, 15 Mar 2019 13:56:39 +0000
> 
> >> I think this should return false if open_source_file fails?
> > 
> > How could it fail if input.is_open returns non-zero?  Are you thinking
> > about some race, whereby something deletes the file after the
> > constructor for 'input' returns?
> 
> Yes, something like that.  The possibility is small, but non-zero
> that it could fail.  And if it does, we end up with s->nlines
> uninitialized again.

OK, pushed with that change to both branches.

Thanks.