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: NIzik Eidus <ieidus@redhat.com>
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch/x86/kvm/mmu.c

@@ -2705,7 +2705,8 @@ EXPORT_SYMBOL_GPL(kvm_mmu_unprotect_page_virt);
 
 void __kvm_mmu_free_some_pages(struct kvm_vcpu *vcpu)
 {
-	while (vcpu->kvm->arch.n_free_mmu_pages < KVM_REFILL_PAGES) {
+	while (vcpu->kvm->arch.n_free_mmu_pages < KVM_REFILL_PAGES &&
+	       !list_empty(&vcpu->kvm->arch.active_mmu_pages)) {
 		struct kvm_mmu_page *sp;
 
 		sp = container_of(vcpu->kvm->arch.active_mmu_pages.prev,