Fix crash when cups returns jobs with duplicate id

Summary:
For some reason my cups was giving me two withheld jobs with id 33 and two with id 40

That made the JobModel code crash, because it went like this

 * First job with id 33 found
 * insertRow with 0 called
 * Row 0 inserted
 * Job at row 0 updated (from inside insertRow)
 * Second job with id 33 found
 * The "oh i already have this job code triggers", updates the job, then takesRow 0 and inserts at row 1. QStandardItemModel doesn't like getting a row add at 1 inserted when empty
 * First job with id 40 found
 * insertRow with 2 called
 * Row 2 inserted, it fails, QStandardItemModel doesn't like getting a row add at 2 when empty
 * Job at row 2 updated (from iniside insertRow)
 * Crash because there's no row 2 in the model

BUGS: 326289

Test Plan: Doesn't crash anymore with my weird cups list of pending jobs

Reviewers: dantti

Reviewed By: dantti

Subscribers: ngraham, marcelm, fvogt, broulik, nicolasfella, kmaterka, kde-utils-devel

Differential Revision: https://phabricator.kde.org/D25623