desktop-shell: Check height instead of checking width a second time

Fix an apparent copy and paste error in resize code. I'm not sure anything
sets the relevant callback that would lead to height being different than
width, so there's no easy way to demonstrate a bug, but this change
appears to rectify the intent of the code.

Reported-by: Hideyuki Nagase <hideyukn@microsoft.com>
Signed-off-by: Derek Foreman <derek.foreman@collabora.com>