klipper: fix invalid image data in clipboard mimedata
Reason for the change
On Wayland images can be only obtained once from some applications. So call QMimeData::imageData() only once to fix the problem.
Test plan
@ndavis : Would you help me create a minimal reproducer?
Screenshots or screen recordings
Bugs fixed
BUG: 497735 FIXED-IN: 6.3
Edited by Fushan Wen