Fix nil pointer dereference in GetPrefixDistributionContext

- Use separate variables for IPv4 and IPv6 query results
- Add nil checks before closing rows to prevent panic
- Prevents crash when database queries timeout or fail

internal/database/database.go

@@ -908,15 +908,19 @@ func (d *Database) GetPrefixDistributionContext(ctx context.Context) (
 		GROUP BY mask_length
 		ORDER BY mask_length
 	`
-	rows, err := d.db.QueryContext(ctx, query)
+	rows4, err := d.db.QueryContext(ctx, query)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to query IPv4 distribution: %w", err)
 	}
-	defer func() { _ = rows.Close() }()
+	defer func() {
+		if rows4 != nil {
+			_ = rows4.Close()
+		}
+	}()
 
-	for rows.Next() {
+	for rows4.Next() {
 		var dist PrefixDistribution
-		if err := rows.Scan(&dist.MaskLength, &dist.Count); err != nil {
+		if err := rows4.Scan(&dist.MaskLength, &dist.Count); err != nil {
 			return nil, nil, fmt.Errorf("failed to scan IPv4 distribution: %w", err)
 		}
 		ipv4 = append(ipv4, dist)
@@ -930,15 +934,19 @@ func (d *Database) GetPrefixDistributionContext(ctx context.Context) (
 		GROUP BY mask_length
 		ORDER BY mask_length
 	`
-	rows, err = d.db.QueryContext(ctx, query)
+	rows6, err := d.db.QueryContext(ctx, query)
 	if err != nil {
 		return nil, nil, fmt.Errorf("failed to query IPv6 distribution: %w", err)
 	}
-	defer func() { _ = rows.Close() }()
+	defer func() {
+		if rows6 != nil {
+			_ = rows6.Close()
+		}
+	}()
 
-	for rows.Next() {
+	for rows6.Next() {
 		var dist PrefixDistribution
-		if err := rows.Scan(&dist.MaskLength, &dist.Count); err != nil {
+		if err := rows6.Scan(&dist.MaskLength, &dist.Count); err != nil {
 			return nil, nil, fmt.Errorf("failed to scan IPv6 distribution: %w", err)
 		}
 		ipv6 = append(ipv6, dist)