libweston/input: Fix assert for valid confine region

We need only check that the region is not empty. If either the input region or
the constraint region have degenerate extents, the intersection from the
previous instruction will set confine_region->data to pixman_region_empty_data.

Fixes: b6423e59

Signed-off-by: Sergio Gómez <sergio.g.delreal@gmail.com>