KVM: MMU: make __kvm_mmu_free_some_pages handle empty list

First check if the list is empty before attempting to look at list
entries.

Cc: stable@kernel.org
Signed-off-by: Izik Eidus <ieidus@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>