avformat/oggenc: check for stream private data in ogg_free()

Fixes a NULL pointer derefence when ogg_init() returns a failure and
a stream's private data was not yet allocated.

This is a regression since 3c5a53cdfa099bba8bd951f95b85727b4b3b5d68

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>
