desktop-shell: fixing null pointer access

There is a chance after certain number of reboots
that ->output is NULL. I assume this is because ->panel
was not yet initialized and is read before written.

Tested on branch weston-imx-6.0.1 from
source.codeaurora.org/external/imx/weston-imx.git
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container_prep

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build-native-meson-default-options

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passed #2426755
build-native-meson-no-gl-renderer

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  Pages
passed #2426756
pages

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  Deploy
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