Fix hole reporting discrepancy.
Fix hole reporting discrepancy.

op_find_initial_pcm_offset() would swallow any holes at the start
 of a link, but seekable streams did not use it when crossing a
 link boundary, so they would report a hole.
This unifies the hole reporting between the two, and also
 simplifies the seeking code (which also needs to ignore holes) a
 bit.

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