fix NoSyncFreelist reachability checking

* scan for free pages if freelist page is valid but the db is opened with
  NoSyncFreelist; forces the freelist pages to count as free pages.

* only treat freelist pages as reachable if set to valid pgid

Fixes boltdb#9

db.go

@@ -242,7 +242,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	}
 
 	db.freelist = newFreelist()
-	noFreeList := db.meta().freelist == pgidNoFreelist
+	noFreeList := db.meta().freelist == pgidNoFreelist || db.NoFreelistSync
 	if noFreeList {
 		// Reconstruct free list by scanning the DB.
 		db.freelist.readIDs(db.freepages())

tx.go

@@ -408,7 +408,7 @@ func (tx *Tx) check(ch chan error) {
 	reachable := make(map[pgid]*page)
 	reachable[0] = tx.page(0) // meta0
 	reachable[1] = tx.page(1) // meta1
-	if !tx.DB().NoFreelistSync {
+	if tx.meta.freelist != pgidNoFreelist {
 		for i := uint32(0); i <= tx.page(tx.meta.freelist).overflow; i++ {
 			reachable[tx.meta.freelist+pgid(i)] = tx.page(tx.meta.freelist)
 		}