Don't crash if the cursor theme fails to create

Summary:
If the cursor theme failed to create KWin crashed due to an endless
recursion. There are two reasons for this fault:
1) When the physical size does not exist we perform a division by 0
which results in an invalid size going into wl_cursor_theme_load
2) We emit the signal that the cursor theme changed even if it didn't
change thus creating an endless recursion

This change addresses both problems: it checks that the size is not 0
and changes the handling for theme update to only destroy the previous
theme if the new theme could be created and only emits the signal if
things change.

BUG: 390314
FIXED-IN: 5.12.3

Test Plan: Added a new test case which crashed with old code

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10549