Avoid panic if creating volume from source fails

If a driver advertises CREATE_DELETE_SNAPSHOT but fails to obey the
part of the spec about allowing CreateVolume from a snapshot (such as
during an incremental development of a driver), the testsuite was
panicking on a nil dereference instead of diagnosing that an
unexpected error was returned.

Fixes: #569
Signed-off-by: Eric Blake <[email protected]>

Author: ebblake

Diff for: pkg/sanity/controller.go

@@ -676,8 +676,9 @@ var _ = DescribeSanity("Controller Service [Controller Server]", func(sc *TestCo
 				},
 			}
 			vol, err := r.CreateVolume(context.Background(), vol2Req)
-			Expect(vol.GetVolume().ContentSource).NotTo(BeNil())
 			Expect(err).NotTo(HaveOccurred())
+			Expect(vol).NotTo(BeNil())
+			Expect(vol.GetVolume().ContentSource).NotTo(BeNil())
 		})
 
 		It("should fail when the volume source snapshot is not found", func() {