Fix an assertion failure.
Fix an assertion failure.

This is no longer guaranteed after the change in e7661191, so test
 for it explicitly.
Thanks to Greg Maxwell for the test case.

SmartWalkieOrg/opusfile,youfearm3/opusfile,youfearm3/opusfile,gcp/opusfile,SmartWalkieOrg/opusfile,xiph/opusfile,gcp/opusfile,xiph/opusfile,gcp/opusfile,xiph/opusfile,SmartWalkieOrg/opusfile