Backport fix for panic when source PVC does not exist

Cherry-pick #381.
Fixed crash of snapshot-controller when source PVC of a snapshot to
take does not exist.

Author: xing-yang

CHANGELOG/CHANGELOG-3.0.md

@@ -10,6 +10,8 @@
 
 - Cherry-pick from https://github.com/kubernetes-csi/external-snapshotter/pull/413. Bug fix to allow creation of snapshot content if pvc finalizer exists, even if pvc is marked for deletion. ([#414](https://github.com/kubernetes-csi/external-snapshotter/pull/414), [@RaunakShah](https://github.com/RaunakShah))
 
+- Cherry-pick from https://github.com/kubernetes-csi/external-snapshotter/pull/381. Fixed crash of snapshot-controller when source PVC of a snapshot to take does not exist. ([#415](https://github.com/kubernetes-csi/external-snapshotter/pull/415), [@jsafrane](https://github.com/jsafrane))
+
 ## Dependencies
 
 ### Added