fix(lib): prevent integer overflow in AddToPort

[yasinlex]

Problem

AddToPort() converts a uint64 value to int without checking for overflow:

newPort := port + int(add)  // int(add) can overflow!

If add is very large (close to math.MaxUint64), int(add) would overflow to a negative number, causing the subsequent bounds check to fail.

Fix

Added overflow check before the conversion:

if add > math.MaxInt64 {
    return "", ErrMaxPort()
}

Impact

• Before: Large add value → integer overflow → incorrect port calculation
• After: Large add value → returns ErrMaxPort() error