hasher: fix invalid memory address error when MaxAge == 0

When f.opt.MaxAge == 0, f.db is never set, however several methods later assume
it is set and attempt to access it, causing an invalid memory address error.
This change fixes the issue in a few spots (there may still be others I haven't
yet encountered.)

lib/kv/bolt.go

@@ -216,7 +216,7 @@ func (db *DB) loop() {
 
 // Do a key-value operation and return error when done
 func (db *DB) Do(write bool, op Op) error {
-	if db.queue == nil {
+	if db == nil || db.queue == nil {
 		return ErrInactive
 	}
 	r := &request{