Update NWWebSocket.swift

[ajinumoto]

Fix multiple NWConnection during migrateConnection

[ajinumoto]

Problem:
Every time migrateConnection() is called, new NWConnection will be created. The problem appear if migrateConnection() is called multiple time, the previous NWConnection from migratedConnection variable is never closed and multiple NWConnection will be alive.

Solution
change migratedConnection to global variable and make sure its cancel() and nil first before creating new one.

[ComeClose322]

if migrateConnection was already called and successfully ended, in the next call when you cancelling migratedConnection you cancelling current 'connection'

[Keith-wright]

Thanks for raising this @ajinumoto. The issue of multiple NWConnection instances being created during migration was addressed in #56, which introduced a guard at the top of migrateConnection that prevents re-entrant calls:

guard !isMigratingConnection else {
    completionHandler(.failure(NWError.posix(.EALREADY)))
    return
}

The same PR also introduced explicit cleanup of the old connection's handlers before cancelling it with a short delay, ensuring no stale callbacks fire during the transition. Closing this in favour of that work.

If you're still able to reproduce the issue on the latest release (0.5.10), please open a new issue and we'll take another look.