KVM: selftests: Fix mostly theoretical leak of VM's binary stats FD

When allocating and freeing a VM's cached binary stats info, check for a
NULL descriptor, not a '0' file descriptor, as '0' is a legal FD. E.g. in
the unlikely scenario the kernel installs the stats FD at entry '0',
selftests would reallocate on the next __vm_get_stat() and/or fail to free
the stats in kvm_vm_free().

Fixes: 83f6e109f562 ("KVM: selftests: Cache binary stats metadata for duration of test")
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

tools/testing/selftests/kvm/lib/kvm_util.c

···
 		return;
 
 	/* Free cached stats metadata and close FD */
-	if (vmp->stats_fd) {
+	if (vmp->stats_desc) {
 		free(vmp->stats_desc);
 		close(vmp->stats_fd);
 	}
···
 	size_t size_desc;
 	int i;
 
-	if (!vm->stats_fd) {
+	if (!vm->stats_desc) {
 		vm->stats_fd = vm_get_stats_fd(vm);
 		read_stats_header(vm->stats_fd, &vm->stats_header);
 		vm->stats_desc = read_stats_descriptors(vm->stats_fd,