Fix potential crash in pastejob
Mimedata is owned by the QPA. It will be deleted when a new clipboard arrives.
PasteJob stores a pointer to the mimedata used. It has an async step
QTimer::singleShot(0, this, SLOT(slotStart()));
mimeData can be
deleted in the meantime.
This patch adds a guard for that.
(I didn't reproduce the original bug, so it's effectively untested)