_initial_pcmoffset uses error codes from vorbis_packet_blocksize in arithmetics
While working on #2140 (closed), I realized that the function _initial_pcmoffset doesn't handle negative values (error codes) returned by vorbis_packet_blocksize properly. It simply uses the returned value in an arithmetic expression, regardless the sign - which leads to an incorrect result. In change, the functions ov_raw_seek and ov_pcm_seek seem to skip the current packet, if vorbis_packet_blocksize returns a negative value.
In the comments of #2140 (closed), tterribe suggested a fix that I'm attaching as patch, but couldn't test yet.
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