KVM: arm64: Tolerate an empty target_table list

Before emptying the target_table lists, and then removing their
infrastructure, add some tolerance to an empty list.

Instead of bugging-out on an empty list, pretend we already
reached the end in the two-list-walk.
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200622113317.20477-3-james.morse@arm.com

arch/arm64/kvm/sys_regs.c

@@ -2809,7 +2809,10 @@ static int walk_sys_regs(struct kvm_vcpu *vcpu, u64 __user *uind)
 	i2 = sys_reg_descs;
 	end2 = sys_reg_descs + ARRAY_SIZE(sys_reg_descs);
 
-	BUG_ON(i1 == end1 || i2 == end2);
+	if (i1 == end1)
+		i1 = NULL;
+
+	BUG_ON(i2 == end2);
 
 	/* Walk carefully, as both tables may refer to the same register. */
 	while (i1 || i2) {