pic: Fix Invalid-enum-value undefined behaviour

Summary:
Instead of directly casting the quint8 to PicChannelEncoding we just store the quint8
and compare it to the possible PicChannelEncoding values when needed

oss-fuzz/19344

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: dfaure, security-team, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25937