fix a bug with variable 'sources' getting an undefined value

We just moved to the latest seaport that uses scuttlebutt in prodution at Browserling and seaport started crashing in a repeated loop:

    /home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/node_modules/scuttlebutt/index.js:212
        sources[source] = ts
                        ^
    TypeError: Cannot set property '58C75509F2D2444F74A4AC93' of undefined
        at Doc.onUpdate (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/node_modules/scuttlebutt/index.js:212:21)
        at Doc.EventEmitter.emit (events.js:115:20)
        at Doc.applyUpdate (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/doc.js:218:8)
        at didVerification (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/node_modules/scuttlebutt/index.js:97:13)
        at Doc.sb._update (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/node_modules/scuttlebutt/index.js:111:5)
        at Doc.sb.localUpdate (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/node_modules/scuttlebutt/index.js:58:8)
        at Row.track (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/doc.js:93:9)
        at Row.EventEmitter.emit (events.js:91:17)
        at Row._set (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/row.js:47:8)
        at Row.set (/home/ec2-user/projects/browserling-seaport-fix/bouncer/node_modules/seaport/node_modules/crdt/row.js:30:8)
    Forever detected script exited with code: 1
    Forever restarting script for 24 time

This patch fixes this problem. Thank you!