Use of invalid regex in schema validator can prevent upgrade

// on 4.0 create a validator with an invalid regex.

db.createCollection("c", {

    validator:

        {email: {$regex: "^*"}}

})

// Now start a 4.2 binary using same data files.

[initandlisten] exception in

    initAndListen: Location51091: Regular expression is invalid: nothing to repeat,

    terminating