Fix custom port input crash

[Pururun]

1. Use toIntOrNull instead of toInt to avoid NumberFormatException.
2. Limit the number of characters in the custom port dialogue input to 5 since numbers that large are not valid port numbers anyway.
3. Add a test that checks for this crash.

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This change is 

[linear]

DROID-294 Crash when custom port input exceeds a certain length or value

Reported by marco. See the attached recording:

Screen_recording_20230831_131649.webm

[albin-mullvad]

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved