Do not skip elements >= (1 << 48) w/o checking for sentinel (#814)

Fixes issue where scan would not terminate when one of the sentinel
values is outside the 48 bit range (shard->params.last >= (1 << 48)).

src/shard.c

@@ -157,10 +157,8 @@ target_t shard_get_cur_target(shard_t *shard)
 
 static inline uint64_t shard_get_next_elem(shard_t *shard)
 {
-	do {
-		shard->current *= shard->params.factor;
-		shard->current %= shard->params.modulus;
-	} while (shard->current >= (1LL << 48));
+	shard->current *= shard->params.factor;
+	shard->current %= shard->params.modulus;
 	return (uint64_t)shard->current;
 }