libweston: Silence tautological-compare warning

libweston/compositor.c:5023:14: warning: comparison of unsigned enum
expression < 0 is always false [-Wtautological-compare]
        if (backend < 0 || backend >= ARRAY_LENGTH(backend_map))
                    ~~~~~~~ ^ ~

Warning produced by Clang 3.8.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>
1 file changed