Fix cob.js Crash

[jpcunningh]

Under certain circumstances when NS has glucose calibration records, cob.js will crash due to avgDelta being undefined.

This PR updates the logic to avoid avgDelta the error causing the crash.

[jpcunningh]

For reference, the crash is:

/root/src/oref0/lib/determine-basal/cob.js:129
        avgDelta = avgDelta.toFixed(2);
                            ^

TypeError: Cannot read property 'toFixed' of undefined
    at detectCarbAbsorption (/root/src/oref0/lib/determine-basal/cob.js:129:29)
    at /root/src/oref0/lib/meal/total.js:68:39
    at Array.forEach (<anonymous>)
    at recentCarbs (/root/src/oref0/lib/meal/total.js:46:16)
    at generate (/root/src/oref0/lib/meal/index.js:20:19)
    at Object.<anonymous> (/root/src/oref0/bin/oref0-meal.js:97:23)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)

[scottleibrand]

Why did you remove this instead of modifying it?

[jpcunningh]

Lines 41-47 were moved lower and lines 48 and 49 were modified to remove the check against the glucose_data[i-1].glucose value and are now lines 47 and 48.

The [i-1] check is no longer required due to how lastbgi is managed.