xwayland: fix error path when modifier is not supported
When the modifier is not supported by the compositor, and the DMA-BUF contains multiple planes, xwl_pixmap->buffer is NULL. Avoid crashing when calling wl_buffer_add_listener().
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When the modifier is not supported by the compositor, and the DMA-BUF contains multiple planes, xwl_pixmap->buffer is NULL. Avoid crashing when calling wl_buffer_add_listener().