Clean up file descriptors on failed database initialization

Fixes an issue where failing to open a non-existent database in ReadOnly
mode would make you unable to properly initialize it in ReadWrite mode
afterwards due to a hanging lock.

db.go

@@ -213,10 +213,13 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 
 	// Initialize the database if it doesn't exist.
 	if info, err := db.file.Stat(); err != nil {
+		_ = db.close()
 		return nil, err
 	} else if info.Size() == 0 {
 		// Initialize new files with meta pages.
 		if err := db.init(); err != nil {
+			// clean up file descriptor on initialization fail
+			_ = db.close()
 			return nil, err
 		}
 	} else {