modetest: Fix segmentation fault

When a mode is set with just a connector "-s foo",
we get a nasty segmentation fault. Fix it.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
1 job for master in 2 minutes and 53 seconds
Status Job ID Name Coverage
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passed #1433682
meson-arch-daily

00:02:53