The metrics structure passed to volume stat calculator
can contain real stats on subset of metrics fields. For example,
the metrics structure filled by a CSI driver can have
either INODES or BYTES filled, IOW it a valid return.
In such cases the volume stat calculator panic with below
trace:

0516 21:36:19.013143   14452 runtime.go:69] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:76
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:65
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:51
/usr/local/go/src/runtime/panic.go:522
/usr/local/go/src/runtime/panic.go:82
/usr/local/go/src/runtime/signal_unix.go:390
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/api/resource/quantity.go:697
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/api/resource/quantity.go:685
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/pkg/kubelet/server/stats/volume_stat_calculator.go:144
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/pkg/kubelet/server/stats/volume_stat_calculator.go:125
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/pkg/kubelet/server/stats/volume_stat_calculator.go:65
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:152
/home/hchiramm/gopath/src/k8s.io/kubernetes/_output/local/go/src/k8s.io/kubernetes/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:153
/usr/local/go/src/runtime/asm_amd64.s:1337
Signed-off-by: Humble Chirammal <hchiramm@redhat.com>