snapshot-controller panics when source PVC does not exist

[jsafrane]

When taking a snapshot of a non-existing PVC, snapshot controller panics:

E0929 06:42:38.147021       1 snapshot_controller_base.go:338] checkAndUpdateSnapshotClass failed to setDefaultClass the snapshot source PVC name is not specified
E0929 06:42:38.147118       1 runtime.go:78] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
goroutine 161 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic(0x1446fc0, 0x201e670)
        /go/src/github.com/kubernetes-csi/external-snapshotter/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:74 +0xa6
k8s.io/apimachinery/pkg/util/runtime.HandleCrash(0x0, 0x0, 0x0)
        /go/src/github.com/kubernetes-csi/external-snapshotter/vendor/k8s.io/apimachinery/pkg/util/runtime/runtime.go:48 +0x89
panic(0x1446fc0, 0x201e670)
        /usr/lib/golang/src/runtime/panic.go:969 +0x175
github.com/kubernetes-csi/external-snapshotter/v3/pkg/common-controller.(*csiSnapshotCommonController).syncSnapshotByKey(0xc0001f8e00, 0xc0006a6ae0, 0x19, 0x0, 0xbc)
        /go/src/github.com/kubernetes-csi/external-snapshotter/pkg/common-controller/snapshot_controller_base.go:215 +0x9d7
github.com/kubernetes-csi/external-snapshotter/v3/pkg/common-controller.(*csiSnapshotCommonController).snapshotWorker(0xc0001f8e00)
        /go/src/github.com/kubernetes-csi/external-snapshotter/pkg/common-controller/snapshot_controller_base.go:188 +0xed
k8s.io/apimachinery/pkg/util/wait.BackoffUntil.func1(0xc0006ba8b0)
        /go/src/github.com/kubernetes-csi/external-snapshotter/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:155 +0x5f
k8s.io/apimachinery/pkg/util/wait.BackoffUntil(0xc0006ba8b0, 0x1774260, 0xc0001f0030, 0x1, 0xc00002a1e0)
        /go/src/github.com/kubernetes-csi/external-snapshotter/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:156 +0xad
k8s.io/apimachinery/pkg/util/wait.JitterUntil(0xc0006ba8b0, 0x0, 0x0, 0x1, 0xc00002a1e0)
        /go/src/github.com/kubernetes-csi/external-snapshotter/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:133 +0x98
k8s.io/apimachinery/pkg/util/wait.Until(0xc0006ba8b0, 0x0, 0xc00002a1e0)
        /go/src/github.com/kubernetes-csi/external-snapshotter/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:90 +0x4d
created by github.com/kubernetes-csi/external-snapshotter/v3/pkg/common-controller.(*csiSnapshotCommonController).Run
        /go/src/github.com/kubernetes-csi/external-snapshotter/pkg/common-controller/snapshot_controller_base.go:139 +0x2ae
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xa0 pc=0x12b1d97]

To reproduce this, the VolumeSnapshot must not have a VolumeSnapshotClass:

apiVersion: snapshot.storage.k8s.io/v1beta1
kind: VolumeSnapshot
metadata:
  name: new-snapshot
spec:
  source:
    persistentVolumeClaimName: blabla

[jsafrane]

/assign