hevc: Correctly set time_base

- Try reading the time_base information from the VPS too, not just the VUI
- Only set time_base when an SPS is activated, not when it's decoded.
- Reduce the fraction before setting it.
- Don't set anything if the fraction is invalid (because the VUI is not
  present or because the encoded value is invalid).

Conflicts:
	libavcodec/hevc_ps.c

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
