libweston: do not forget to check if test_data is NULL

Since commit "libweston: add required_capabilities test suite quirk"
a new function that depends on test_data is being called in wet_main().

We should check if test_data is NULL before calling it, otherwise
we have a segfault when running outside the test suite.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

compositor/main.c

@@ -3382,7 +3382,7 @@ wet_main(int argc, char *argv[], const struct weston_testsuite_data *test_data)
 		goto out;
 	}
 
-	if (!check_compositor_capabilities(wet.compositor,
+	if (test_data && !check_compositor_capabilities(wet.compositor,
 				test_data->test_quirks.required_capabilities)) {
 		ret = WET_MAIN_RET_MISSING_CAPS;
 		goto out;