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

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/20250111005049.1247555-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>