Zero amount handling in engine.js

[tuxitor]

engine.js bails out if zero values are received

2017-06-23 02:24:44  0.28 XRP-USD   -0.4%    98467    --         0.30%             0.00 XRP   0.00 USD/home/eigilb/Prosjekt/Z/work/node_modules/numbro/numbro.js:1111
        var parts = x.toString().split('.');
                     ^

TypeError: Cannot read property 'toString' of undefined
    at multiplier (/home/eigilb/Prosjekt/Z/work/node_modules/numbro/numbro.js:1111:22)
    at /home/eigilb/Prosjekt/Z/work/node_modules/numbro/numbro.js:1127:22
    at Array.reduce (native)
    at correctionFactor (/home/eigilb/Prosjekt/Z/work/node_modules/numbro/numbro.js:1125:21)
    at cback (/home/eigilb/Prosjekt/Z/work/node_modules/numbro/numbro.js:1218:34)
    at Array.reduce (native)
    at Object.multiply (/home/eigilb/Prosjekt/Z/work/node_modules/numbro/numbro.js:1224:48)
    at writeReport (/home/eigilb/Prosjekt/Z/work/lib/engine.js:715:72)
    at Timeout._onTimeout (/home/eigilb/Prosjekt/Z/work/lib/engine.js:633:11)
    at ontimeout (timers.js:386:14)
    at tryOnTimeout (timers.js:250:5)
    at Timer.listOnTimeout (timers.js:214:5)

[haxwell]

I think this is the same as #457, which has been resolved.