Fix crash when triggereing the "Compare files" action via D-Bus

Fix crash when triggereing the "Compare files" action via D-Bus

If the number of selected items is not two, Dolphin disables this
action. However, it is still possible to trigger it via D-Bus, and this
could cause a crash in DolphinMainWindow::compareFiles() because this
function did not test at all if there are really two items selected.

This patch adds such a check and simplifies the code in that function.

BUG: 325517
FIXED-IN: 4.11.3