weston: Properly test for output-creation failure

We were testing the wrong variable to see if output creation had failed:
instead of testing the return of the function we'd just called, we were
testing something we'd already checked earlier.

Signed-off-by: Daniel Stone <daniels@collabora.com>
(cherry picked from commit 81433582)
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passed #368366
build-native-autotools

00:04:15

passed #368367
build-native-meson

00:02:48