Before alter group make sure user still exists

When there is no dependency between user and group or user is in
multiple groups and was removed before then terraform will fail. After this
commit we fix problem and during alter table we check if user still
exists in redshift.

redshift/resource_redshift_group.go

@@ -213,6 +213,26 @@ func setGroupName(tx *sql.Tx, d *schema.ResourceData) error {
 	return nil
 }
 
+func checkIfUserExists(tx *sql.Tx, name interface{}) (bool, error) {
+
+	username, ok := name.(string)
+	if !ok {
+		return false, fmt.Errorf("cannot convert username to string")
+	}
+
+	var result int
+	err := tx.QueryRow("SELECT 1 from pg_user_info WHERE usename=$1", username).Scan(&result)
+
+	switch err {
+	case sql.ErrNoRows:
+		return false, nil
+	case nil:
+		return false, fmt.Errorf("error reading info about user: %s", err)
+	}
+
+	return true, nil
+}
+
 func setUsersNames(tx *sql.Tx, db *DBConnection, d *schema.ResourceData) error {
 	if !d.HasChange(groupUsersAttr) {
 		return nil
@@ -226,13 +246,22 @@ func setUsersNames(tx *sql.Tx, db *DBConnection, d *schema.ResourceData) error {
 	if removedUsers.Len() > 0 {
 		removedUsersNamesSafe := []string{}
 		for _, name := range removedUsers.List() {
-			removedUsersNamesSafe = append(removedUsersNamesSafe, pq.QuoteIdentifier(name.(string)))
+			userExists, err := checkIfUserExists(tx, name)
+			if err != nil {
+				return err
+			}
+
+			if userExists {
+				removedUsersNamesSafe = append(removedUsersNamesSafe, pq.QuoteIdentifier(name.(string)))
+			}
 		}
 
-		sql := fmt.Sprintf("ALTER GROUP %s DROP USER %s", pq.QuoteIdentifier(groupName), strings.Join(removedUsersNamesSafe, ", "))
+		if len(removedUsersNamesSafe) > 0 {
+			sql := fmt.Sprintf("ALTER GROUP %s DROP USER %s", pq.QuoteIdentifier(groupName), strings.Join(removedUsersNamesSafe, ", "))
 
-		if _, err := tx.Exec(sql); err != nil {
-			return err
+			if _, err := tx.Exec(sql); err != nil {
+				return err
+			}
 		}
 	}