Fix a granpos calculation we assumed couldn't fail.
Fix a granpos calculation we assumed couldn't fail.

It was only guaranteed to work in the seekable case.
Thanks to Radio Stadtfilter 96.3Mhz for sending a stream which
 triggered this failure.

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