bug#9948: valgrind warning: Conditional jump or move depends on uninitialised value(s) in redisplay_window

[Eli Zaretskii]

> Date: Sun, 06 Nov 2011 22:08:54 -0800
> From: Paul Eggert <eggert@cs.ucla.edu>
> CC: 9948@debbugs.gnu.org
> 
>   SET_TEXT_POS_FROM_MARKER (startp, w->start);
> 
> does not set startp to a valid charpos if w->start
> points to uninitialized data.

w->startp is the window-start of the window represented by `w'.  It is
a very important player in displaying the window.  If that is
uninitialized, we have a much larger catastrophe on our hands than
some obscure local variable that _could_ be uninitialized ;-)

IOW, if w->startp does not have a valid value, Emacs will crash
pronto.