Wait to clear db.path until just before db.close returns.

The Windows version of funlock needs the db.path to delete the
corresponding .lock file.

db.go

@@ -387,7 +387,6 @@ func (db *DB) close() error {
 	db.opened = false
 
 	db.freelist = nil
-	db.path = ""
 
 	// Clear ops.
 	db.ops.writeAt = nil
@@ -414,6 +413,7 @@ func (db *DB) close() error {
 		db.file = nil
 	}
 
+	db.path = ""
 	return nil
 }