fix sorting

[richardlau]

#289 broke sorting as it removed the assignment to cols which is referenced later on

istanbuljs/packages/istanbul-reports/lib/html/assets/sorter.js

Line 61 in 9dac4c6

col = cols[i];

In a web browser loading the html report gives the following in the web console:

TypeError: cols is undefined[Learn More] sorter.js:62:13
    loadRowData https://coverage.nodejs.org/coverage-09cdc3782488d2e6/sorter.js:62
    loadData https://coverage.nodejs.org/coverage-09cdc3782488d2e6/sorter.js:77
    addSorting https://coverage.nodejs.org/coverage-09cdc3782488d2e6/sorter.js:164

cc @Trott

[coreyfarrell]

@richardlau thanks for the patch, merged and published to istanbul-reports@2.2.3.