Not checking if account is a multisign kill node

[fix]

[log] 2017-01-09 14:23:54 | POST /api/multisignatures/sign from 127.0.0.1
[FTL] 2017-01-09 14:23:54 | System error - {"message":"Cannot read property 'indexOf' of null","stack":"TypeError: Cannot read property 'indexOf' of null\n at Immediate. (/Users/vsat/git/lisk/lisk-node/modules/multisignatures.js:371:33)\n at runCallback (timers.js:641:20)\n at tryOnImmediate (timers.js:610:5)\n at processImmediate [as _immediateCallback] (timers.js:582:5)"}
[inf] 2017-01-09 14:23:54 | Cleaning up...
[inf] 2017-01-09 14:23:54 | Cleaned up successfully
[inf] 2017-01-09 14:23:54 | caught internal exit

was using old version of lisk, but the bug remain:

https://github.com/LiskHQ/lisk/blob/development/modules/multisignatures.js#L360

scope.sender.multisignatures is not checked if exists. So trying to sign as multi over a legit account which is not a multiaccount is killing the node