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