test: add one more assertion to make Coverty happy

Fixes CID#1452934.

src/test/test-strbuf.c

@@ -29,6 +29,7 @@ static void test_strbuf(void) {
 
         /* check the content of the buffer directly */
         l = strv_parse_nulstr(sb->buf, sb->len);
+        assert_se(l);
 
         assert_se(streq(l[0], "")); /* root */
         assert_se(streq(l[1], "waldo"));