input_verifier: check the number of pointers for MOVE events

Before this CL, the check for the existing pointers was not complete:
there was a chance that the MOVE event contained less than the current
number of pointers.

Fix this by checking the number of pointers before comparing the sets of
pointers.

Bug: 312714754
Test: atest libinput_rust_test
Change-Id: I4e6c00aec4247b6cb4c467ac5c2887ee3a90afba