KVM: x86: avoid vmalloc(0) in the KVM_SET_CPUID

This causes an ugly dmesg splat.  Beautified syzkaller testcase:

    #include <unistd.h>
    #include <sys/syscall.h>
    #include <sys/ioctl.h>
    #include <fcntl.h>
    #include <linux/kvm.h>

    long r[8];

    int main()
    {
        struct kvm_irq_routing ir = { 0 };
        r[2] = open("/dev/kvm", O_RDWR);
        r[3] = ioctl(r[2], KVM_CREATE_VM, 0);
        r[4] = ioctl(r[3], KVM_SET_GSI_ROUTING, &ir);
        return 0;
    }
Reported-by: NDmitry Vyukov <dvyukov@google.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>

virt/kvm/kvm_main.c

@@ -2935,7 +2935,7 @@ static long kvm_vm_ioctl(struct file *filp,
 	case KVM_SET_GSI_ROUTING: {
 		struct kvm_irq_routing routing;
 		struct kvm_irq_routing __user *urouting;
-		struct kvm_irq_routing_entry *entries;
+		struct kvm_irq_routing_entry *entries = NULL;
 
 		r = -EFAULT;
 		if (copy_from_user(&routing, argp, sizeof(routing)))
@@ -2945,6 +2945,7 @@ static long kvm_vm_ioctl(struct file *filp,
 			goto out;
 		if (routing.flags)
 			goto out;
+		if (routing.nr) {
 			r = -ENOMEM;
 			entries = vmalloc(routing.nr * sizeof(*entries));
 			if (!entries)
@@ -2954,6 +2955,7 @@ static long kvm_vm_ioctl(struct file *filp,
 			if (copy_from_user(entries, urouting->entries,
 					   routing.nr * sizeof(*entries)))
 				goto out_free_irq_routing;
+		}
 		r = kvm_set_irq_routing(kvm, entries, routing.nr,
 					routing.flags);
 out_free_irq_routing: