wheelhandler: Prevent division by zero
Before anyone says this can't happen:
#10 0x00007f2868125b95 in WheelHandler::startInertiaScrolling (this=0x55953b7eaac0) at /home/apol/devel/frameworks6/kirigami/src/wheelhandler.cpp:464
464 QPointF vel = totalDelta * 1000 / elapsed * speedFactor;
(gdb) p elapsed
$1 = 0