Fix int overflow when parsing ImapSet

Summary:
The ImapSet is a vector of qint64s, but while iteraring over
the vector the code stored the value as an int, which could've
caused overflow if the Id was large enough, triggering the
Q_ASSERT below.

BUG: 409419
FIXED-IN: 5.11.3

Reviewers: #kde_pim, vkrause

Reviewed By: #kde_pim, vkrause

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22271