Fix Create from snapshot test

[ebblake]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Fix a panic from a nil dereference when diagnosing an incomplete driver that supports snapshot creation but not volume creation from a snapshot.

Which issue(s) this PR fixes:

Fixes #569

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ebblake
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[k8s-ci-robot]

Hi @ebblake. Thanks for your PR.

I'm waiting for a kubernetes-csi member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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