KVM: page_track: fix access to NULL slot

This happens when doing the reboot test from virt-tests:

[  131.833653] BUG: unable to handle kernel NULL pointer dereference at           (null)
[  131.842461] IP: [<ffffffffa0950087>] kvm_page_track_is_active+0x17/0x60 [kvm]
[  131.850500] PGD 0
[  131.852763] Oops: 0000 [#1] SMP
[  132.007188] task: ffff880075fbc500 ti: ffff880850a3c000 task.ti: ffff880850a3c000
[  132.138891] Call Trace:
[  132.141639]  [<ffffffffa092bd11>] page_fault_handle_page_track+0x31/0x40 [kvm]
[  132.149732]  [<ffffffffa093380f>] paging64_page_fault+0xff/0x910 [kvm]
[  132.172159]  [<ffffffffa092c734>] kvm_mmu_page_fault+0x64/0x110 [kvm]
[  132.179372]  [<ffffffffa06743c2>] handle_exception+0x1b2/0x430 [kvm_intel]
[  132.187072]  [<ffffffffa067a301>] vmx_handle_exit+0x1e1/0xc50 [kvm_intel]
...

Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Fixes: 3d0c27ad6ee465f174b09ee99fcaf189c57d567a
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/page_track.c

@@ -142,12 +142,17 @@ void kvm_slot_page_track_remove_page(struct kvm *kvm,
 bool kvm_page_track_is_active(struct kvm_vcpu *vcpu, gfn_t gfn,
 			      enum kvm_page_track_mode mode)
 {
-	struct kvm_memory_slot *slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
-	int index = gfn_to_index(gfn, slot->base_gfn, PT_PAGE_TABLE_LEVEL);
+	struct kvm_memory_slot *slot;
+	int index;
 
 	if (WARN_ON(!page_track_mode_is_valid(mode)))
 		return false;
 
+	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
+	if (!slot)
+		return false;
+
+	index = gfn_to_index(gfn, slot->base_gfn, PT_PAGE_TABLE_LEVEL);
 	return !!ACCESS_ONCE(slot->arch.gfn_track[mode][index]);
 }