A server's player limit can be set to 0 or less
Steps to Reproduce:
1) Open server.properties
2) Set max-players to -1 and save
3) Start the server
Expected:
The server doesn't start
Observed:
The server saves its player limit as -1
Created Issue:
A server's player limit can be set to 0 or less
Steps to Reproduce:
1) Open server.properties
2) Set max-players to -1 and save
3) Start the serverExpected:
The server player limit resets to the default 20Observed:
The server saves its player limit as -1
Steps to Reproduce:
1) Open server.properties
2) Set max-players to -1 and save
3) Start the serverExpected:
The serverplayer limit resets to the default 20Observed:
The server saves its player limit as -1Steps to Reproduce:
1) Open server.properties
2) Set max-players to -1 and save
3) Start the serverExpected:
The server doesn't startObserved:
The server saves its player limit as -1

I'd say resetting back to 20 would be odd, logging an error and closing the server would make more sense. (Just like when EULA flag isn't set to true.)
Expected outcome should be that server does not start