Copy root field from wire event into root, not window, of Xlib event

Fixes cppcheck warning:
[src/XScrnSaver.c:104] -> [src/XScrnSaver.c:105]: (performance, inconclusive)
 Variable 'window' is reassigned a value before the old one has been
 used if variable is no semaphore variable.

Error seems to date back to revision 1.1 in X Consortium RCS,
dated 1992-02-13, by "keith"

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>