Stream timebase should be set using avpriv_set_pts_info, otherwise
avctx->pkt_timebase is not correct.

This should fix ticket #4766.

Patch is only compile tested.
Signed-off-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Stephen Hutchinson <qyot27@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>