Appease Coverity

CID#469261

src/slashmap.h

@@ -91,6 +91,7 @@ next_freepage(const struct slashmap *map, struct mappage at, size_t *cache)
 	j = (int)map->max - from.level;
 	if (i > j)
 		i = j;
+	assert(i >= 0);
 	from.level += i;
 	from.page >>= i;
 	assert(from.level >= (int)map->min);