viewporter: fix "has buffer" check
Previously, cases when a viewport source box was updated without attaching a buffer weren't checked for correctness properly because the state's buffer would be NULL. Use wlr_surface_state_has_buffer() instead.
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Previously, cases when a viewport source box was updated without attaching a buffer weren't checked for correctness properly because the state's buffer would be NULL. Use wlr_surface_state_has_buffer() instead.