Better password hashes error handling (#132)

internal/web/util.go

@@ -265,8 +265,16 @@ func (a Argon2ID) hash(plain string) (string, error) {
 }
 
 func (a Argon2ID) verify(plain, hash string) (bool, error) {
+	if hash == "" {
+		return false, nil
+	}
+
 	hashParts := strings.Split(hash, "$")
 
+	if len(hashParts) != 6 {
+		return false, errors.New("invalid hash")
+	}
+
 	_, err := fmt.Sscanf(hashParts[3], "m=%d,t=%d,p=%d", &a.memory, &a.time, &a.threads)
 	if err != nil {
 		return false, err