xwm: Fix accidental resizing of windows

If an X window has not been resized yet, and the user clicks (not drags)
on a part of the window frame that can be used to resize, the window
will resize to 512, 512.

This is likely because of the changes in ba82af93 and 2acd2c74
that change resize behaviour but missed updating saved size when the
client's initial configure occurs.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>