layer-shell: Fix crash when cursor is intially outside any output
layer-shell: Fix crash when cursor is intially outside any output

On the X11 backend the cursor position might be outside the output
window so no output is returned leading to the assert to trigger. Use
sane fallback instead of crashing.

SirCmpwn/wlroots,swaywm/wlroots,ascent12/wlroots,ascent12/wlroots,swaywm/wlroots