We need a check here, because stickers (and really, any images without a
tempSource) will try to access an undefined object.
This fixes the error:
"qrc:/qt/qml/org/kde/neochat/timeline/ImageComponent.qml:106: TypeError: Cannot read property 'source' of undefined"
(cherry picked from commit 2a8cd74ab1)
If we do not set the width/height for stickers (which don't have any)
then the height is okay, but the message has the maximum width which
looks odd.
Instead, let's limit all stickers to 256px and it makes them look much
nicer in chat.
(cherry picked from commit 63bc7055c2)
