universal-access: Free zoom options dialog right

Use gtk_widget_destroy() instead of g_clear_object().
Otherwise, it results in segfault on dispose()
11 jobs for wip/sadiq/ua-zoom-fix in 4 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #354899
build

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  Test
passed #354901
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passed #354900
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  Manual
manual #354906
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armv8
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flatpak
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ppc64le
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tsan
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  Review
manual #354908
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review
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